Science, Faculty of
Chemistry, Department of
DSpace
UBCV
Yang, Ruijie
Master of Science - MSc
University of British Columbia
Rubber is a widely used material that plays an essential role in our daily life. However, the challenging reaction conditions make butyl rubber manufacturing very energy-intensive and difficult to control. Brønsted acids are efficient single component initiators for cationic polymerization with a simple functional cation, H⁺. The coordinating force between cations and anions is crucial for developing high-performance initiators. Weakly coordinating anions have moderate interaction force with cations. This thesis will focus on developing functional initiators containing Brønsted acids and weakly coordinating anions. The growth of the field of weakly coordinating anions has mainly relied on p-block elements such as boron, aluminum, phosphorus, and sulfur. Transition metal complexes have rarely been used as weakly coordinating anions. This thesis will show efforts on designing Ta(V)- or Nb(V)-containing steric bulky WCAs for stabilizing functional Brønsted acids. Chapters 2 and chapter 3 have shown eleven cationic initiators. Among all these cationic initiators, two of them contain anions featuring heptacoordinated Ta(V) with pentagonal bipyramid geometry. These are unprecedented Brønsted acids. H(DMF)₂[2.1] contains the [Ta(1,2-O₂C₆Cl₄)₂(1,2–O₂(OH)₂C₆Cl₄)₂(O=CHN(CH₃)₂)]– anion which has two chelated tetrachlorocatechol ligands, two monodentate tetrachlorocatechol ligands and another DMF ligand with an overall charge of was –1. In addition, [H(DMA)]₂[2.2] was isolated and crystallographically characterized. This Brønsted acid features the novel [Ta(1,2-O₂C₆Cl₄)₃(1,2– O₂(OH)₂C₆Cl₄)]²– dianion with three chelated tetrachlorocatechol and only one monodentate tetrachlorocatechol. Although not specifically tested herein, these new Brønsted acids form a foundation for the future design of novel initiators for cationic polymerization
Brønsted Acids with Weakly Coordinating Anions Featuring Group 5 MetalsbyRuijie YangB.Eng. Xiamen University, 2018A THESIS SUBMITTED IN PARTIAL FULFILLMENT OFTHE REQUIREMENTS FOR THE DEGREE OFMASTER OF SCIENCEinTHE FACULTY OF GRADUATE AND POSTDOCTORAL STUDIES(Chemistry)THE UNIVERSITY OF BRITISH COLUMBIA(Vancouver)November 2021© Ruijie Yang, 2021iiThe following individuals certify that they have read, and recommend to the Faculty of Graduateand Postdoctoral Studies for acceptance, a thesis entitled:Brønsted acids with weakly coordinating anions featuring group 5 metalssubmitted by Ruijie Yang in partial fulfillment of the requirements forthe degree of Master of Sciencein ChemistryExamining Committee:Derek. Gates, Professor, Department of Chemistry, UBCSupervisorParisa Mehrkhodavandi, Professor, Department of Chemistry, UBCSupervisory Committee MemberChris Orvig, Professor, Department of Chemistry, UBCSupervisory Committee MemberiiiAbstractRubber is a widely used material that plays an essential role in our daily life. However,the challenging reaction conditions make butyl rubber manufacturing very energy-intensive anddifficult to control. Brønsted acids are efficient single component initiators for cationicpolymerization with a simple functional cation, H+. The coordinating force between cations andanions is crucial for developing high-performance initiators. Weakly coordinating anions havemoderate interaction force with cations. This thesis will focus on developing functional initiatorscontaining Brønsted acids and weakly coordinating anions.The growth of the field of weakly coordinating anions has mainly relied on p-blockelements such as boron, aluminum, phosphorus, and sulfur. Transition metal complexes haverarely been used as weakly coordinating anions. This thesis will show efforts on designingTa(V)- or Nb(V)-containing steric bulky WCAs for stabilizing functional Brønsted acids.Chapters 2 and chapter 3 have shown eleven cationic initiators. Among all these cationicinitiators, two of them contain anions featuring heptacoordinated Ta(V) with pentagonalbipyramid geometry. These are unprecedented Brønsted acids. H(DMF)2[2.1] contains the[Ta(1,2-O2C6Cl4)2(1,2–O2(OH)2C6Cl4)2(O=CHN(CH3)2)]– anion which has two chelatedtetrachlorocatechol ligands, two monodentate tetrachlorocatechol ligands and another DMFligand with an overall charge of was –1. In addition, [H(DMA)]2[2.2] was isolated andcrystallographically characterized. This Brønsted acid features the novel [Ta(1,2-O2C6Cl4)3(1,2–O2(OH)2C6Cl4)]2– dianion with three chelated tetrachlorocatechol and only one monodentatetetrachlorocatechol. Although not specifically tested herein, these new Brønsted acids form afoundation for the future design of novel initiators for cationic polymerization.ivLay SummaryPolymers are synthetic or natural large molecules that include repetitive units known asmonomers. Synthetic polymerization reactions are usually performed in three steps: initiation,propagation, and termination. Initiation is the first step for starting a polymerization reaction, andit requires both an initiating species and a monomer.Butyl rubber can only be manufactured using a method known as cationic polymerization.Cations are positively charged ions are used to initiate a cationic polymerization reaction.Building effective cationic initiators necessitates a focus not just on cations but also on counter-anion. Only below -90 °C can these initiators polymerize the monomers. The extremely lowtemperature will cost a lot of energy and money. The goal of this thesis is to create someinitiators with a moderate cation-anion interaction, so that these initiators may be used in a moremoderate environment.vPrefaceI synthesized and analyzed all of the chemicals and polymers described in this thesis. Dr.Brian O. Patrick performed the X-ray crystallographic experiments and solved the crystalstructures reported in Chapters 2 and 3. As a result of the inaccessibility to NMR spectrometersfor hands on use during the pandemic, some NMR spectra were recorded by technical staff in theUBC Chemistry NMR Facility and I interpreted the spectra. I wrote all of the chapters, and thecontents in Chapters 2 and 3 may be considered for publication in polymer-related publications.viTable of ContentsAbstract......................................................................................................................................... iiiLay Summary................................................................................................................................ ivPreface.............................................................................................................................................vTable of Contents..........................................................................................................................viList of Tables.................................................................................................................................. xList of Figures................................................................................................................................xiList of Schemes............................................................................................................................xivList of Abbreviations................................................................................................................... xvAcknowledgments...................................................................................................................... xxiiChapter 1: Introduction................................................................................................................ 11.1 Rubber History.......................................................................................................................11.2 Cationic Polymerization.........................................................................................................31.2.1 Cationic Polymerization General Mechanism................................................................ 31.2.2 Properties and Influential Factors of Cationic Polymerization.......................................51.3 Initiators Used in Cationic Polymerization............................................................................81.3.1 Binary Initiators.............................................................................................................. 81.3.2 Single-Component Initiators for Cationic Polymerization........................................... 101.4 Weakly Coordinating Anions (WCAs)................................................................................101.4.1 B-centered WCAs......................................................................................................... 121.4.2 Al-centered WCAs........................................................................................................151.4.3 Other Main Group Elements Centered WCAs..............................................................16vii1.5 Brønsted Acids with WCAs.................................................................................................181.6 Outline of the Thesis............................................................................................................19Chapter 2: Single Component Cationic Initiators with Hexacoordinate andHeptacoordinate Ta(V)-containing Weakly coordinating Anions.......................................... 212.1 Introduction..........................................................................................................................212.2 Results and Discussion about cationic initiators with Ta-centered WCAs..........................252.2.1 [H(DMF)2][2.1].............................................................................................................252.2.2 Crystal Structure Information with Metrical Parameters..............................................292.2.3 [H(DMA)]2[2.2]............................................................................................................ 342.2.4 Crystal Structure Information with Metrical Parameters..............................................372.2.5 [H(DMA)][2.4]............................................................................................................. 412.2.6 H(THF)2[2.3]................................................................................................................ 432.2.7 H(THF)2][2.4]............................................................................................................... 462.2.8 H(Et2O)2[2.3]................................................................................................................ 482.2.9 H(Et2O)2[2.4]................................................................................................................ 502.2.10 Summary.....................................................................................................................522.3 Experimental Procedures..................................................................................................... 522.3.1 The General Procedures................................................................................................522.3.2 The H(DMF)2[2.1] Synthesis Procedure.......................................................................542.3.3 The H(DMF)2[2.1] X-ray Structure Procedure............................................................ 552.3.4 The [H(DMA)]2[2.2] Synthesis Procedure................................................................... 582.3.5 The [H(DMA)]2[2.2] X-ray Structure Procedure..........................................................59viii2.3.6 The H(DMA)[2.4] Synthesis Procedure....................................................................... 612.3.7 The H(THF)2[2.3] Synthesis Procedure........................................................................622.3.8 The H(THF)2[2.4] Synthesis Procedure........................................................................632.3.9 The H(Et2O)2[2.3] Synthesis Procedure........................................................................642.3.10 The H(Et2O)2[2.4] Synthesis Procedure......................................................................65Chapter 3: Single Component Cationic Initiators for Olefin polymerization withHexacoordinate and Heptacoordinate Niobium(V) Weakly Coordinating Anions...............673.1 Introduction..........................................................................................................................673.2 Results and Discussion........................................................................................................ 703.2.1 Synthesis of [H(DMA)][3.1].........................................................................................703.2.2 Synthesis of [H(aniline)][3.1]....................................................................................... 733.2.3 Synthesis of [H(DMA)][3.2].........................................................................................753.2.4 Synthesis of [H(aniline)][3.2]....................................................................................... 773.2.5 Summary.......................................................................................................................793.3 Experimental........................................................................................................................ 803.3.1 The General Procedures................................................................................................803.3.2 Synthesis of H(DMA)[3.1]........................................................................................... 813.3.3 Synthesis of H(aniline)[3.1]..........................................................................................823.3.4 Synthesis of H(DMA)[3.2]........................................................................................... 833.3.5 Synthesis of H(aniline)[3.2]..........................................................................................84Chapter 4: Conclusion.................................................................................................................864.1 Introduction..........................................................................................................................86ix4.2 Group 5 Hexacoordinate and Heptacoordinated WCAs......................................................86Bibliography................................................................................................................................. 89Appendix.....................................................................................................................................110xList of TablesTable 2.1 X-ray crystallographic parameters for H(DMF)2[2.1]...................................................55Table 2.2 X-ray crystallographic parameters for [H(DMA)]2[2.2]............................................... 59xiList of FiguresFigure 1.1 The structure of natural rubber’s main components, cis-polyisoprene.......................... 2Figure 1.2 Four different solvent effect types................................................................................. 6Figure 1.3 Binary system mechanism..............................................................................................9Figure 1.4 1). Interaction between cations and small, nucleophilic anions 2). Interactionbetween cations and weakly coordinating anions (WCAs)........................................................... 12Figure 1.5 Structures of B-centered WCAs...................................................................................14Figure 1.6 Structure of Al-centered WCAs................................................................................... 16Figure 1.7 Other WCAs containing p-block elements...................................................................18Figure 1.8 The two single-component initiators applied in cationic polymerization.................... 19Figure 2.1Structure of three traditional P-centered WCAs........................................................... 22Figure 2.2 Structures of P-centered single component initiator from our group...........................22Figure 2.3Structures of traditional Ta-centered WCAs.................................................................23Figure 2.4 Structures of Ta-centered single component initiator from our group.........................24Figure 2.5 Room temperature 1H NMR (400 MHz, CDCl3) spectrum of [H(DMF)][2.1]............27Figure 2.6 Low temperature (-85 °C) 1H NMR (400 MHz, CDCl3) spectrum of [H(DMF)2][2.1]........................................................................................................................................................ 28Figure 2.7 The single crystal structure of H(DMF)2[Ta(V)-cis-(1,2-O2C6Cl4)2-trans-(1,2–O2(OH)2C6Cl4)2(O=CHN(CH3)2)]·0.7CH2Cl2 is demonstrated.....................................................29Figure 2.8 The two monodentate C6Cl4O(OH)- ligands moieties are bent towards the DMFmolecule.........................................................................................................................................31xiiFigure 2.9 The sketch of the center structure of H(DMF)2[2.1].................................................... 33Figure 2.10 Metrical parameters [Å] for the cation in H(DMF)2[Ta-cis-(1,2-O2C6Cl4)2-trans-(1,2–O2(OH)2C6Cl4)2(O=CHN(CH3)2)]·0.7CH2Cl2............................................................................... 34Figure 2.11 1H NMR (400 MHz, CD2Cl2, Room Temperature) spectrum of [H(DMA)]2[2.2].... 36Figure 2.12 1H NMR (400 MHz, CD2Cl2, -85 °C) spectrum of [H(DMA)]2[2.2].........................37Figure 2.13 The single crystal structure of [H(DMA)]2[Ta-cis-(1,2-O2C6Cl4)3-(1,2–O2(OH)2C6Cl4)1]·0.5CH2Cl2 is demonstrated................................................................................ 38Figure 2.14 the sketch of the center structure of [H(DMA)]2[2.2]................................................ 40Figure 2.15 Metrical parameters [Å] for the two cations of [H(DMA)]2[Ta-(1,2-O2C6Cl4)3-(1,2–O2(OH)2C6Cl4)]·0.5CH2Cl2........................................................................................................... 41Figure 2.16 1H NMR (400 MHz, CD2Cl2, Room Temperature) spectrum of [H(DMA)][2.4]..... 43Figure 2.17 1H NMR (400 MHz, CD2Cl2, –85 °) spectrum of H(THF)2[2.3]............................... 45Figure 2.18 1H NMR (400 MHz, CD2Cl2, –85 °) spectrum of H(THF)2[2.4]............................... 47Figure 2.19 1H NMR (400 MHz, CD2Cl2, –85 °C) spectrum of H(OEt2)2[2.3]............................ 49Figure 2.20 1H NMR (400 MHz, CD2Cl2, –85 °C) spectrum of H(OEt2)2[2.4]...........................51Figure 3.1 Structures of traditional Nb-centered Single component initiators..............................69Figure 3.2 Structures of Nb-centered single component initiators synthesized by our group.......70Figure 3.3 1H NMR (400 MHz, CD2Cl2, Room Temperature) spectrum of H(DMA)[3.1].........72Figure 3.4 1H NMR (400 MHz, CD2Cl2, Room Temperature) spectrum of H(aniline)[3.1]....... 75Figure 3.5 1H NMR (400 MHz, CD2Cl2, Room Temperature) spectrum of H(DMA)[3.2].......... 77Figure 3.6 1H NMR (400 MHz, CD2Cl2, Room Temperature) spectrum of H(aniline)[3.2].........79xiiiFigure A.1 1H–1H COSY NMR (400 MHz for 1H, CDCl3, 25 °C) spectrum of [H(DMA)]2[2.2]......................................................................................................................................................110Figure A.2 LIFDI mass spectrum of [3.1]- part A....................................................................... 111Figure A.3 LIFDI mass spectrum of [3.1]- part B....................................................................... 112Figure A.4 LIFDI mass spectrum of [3.2]- part A....................................................................... 113Figure A.5 LIFDI mass spectrum of [3.2]- part B....................................................................... 114Figure A.6 13C{1H} NMR (400 MHz, CDCl3, 25 °C) spectrum of H(DMA)[3.2]..................... 115Figure A.7 1H–1H COSY NMR (400 MHz for 1H, CDCl3, 25 °C) spectrum of [H(DMA)][3.1]...................................................................................................................................................... 116Figure A.7 1H–1H COSY NMR (400 MHz for 1H, CDCl3, 25 °C) spectrum of [H(DMA)][3.2]...................................................................................................................................................... 117xivList of SchemesScheme 1.1 General mechanism of cationic chain polymerization.................................................4Scheme 1.2 Three different interactions between the cationic parts and anionic part, and the sideeffects of the other interactions for cationic polymerization......................................................... 11Scheme 2.1 Equilibrium of solvated proton and counterions from Brønsted acids...................... 21Scheme 2.2 Procedures for synthesizing Brønsted acid H(DMF)2[2.1]........................................ 26Scheme 2.3 Procedures for synthesizing Brønsted acid [H(DMA)]2[2.2].....................................35Scheme 2.4 Synthesis of Brønsted acid [H(DMA)]2[2.4]............................................................. 42Scheme 2.5 Synthesis of Brønsted acid H(THF)2[2.3]..................................................................44Scheme 2.6 Synthesis of Brønsted acid H(THF)2[2.4]..................................................................46Scheme 2.7 Synthesis of Brønsted acid H(OEt2)2[2.3]..................................................................48Scheme 2.8 Synthesis of Brønsted acid H(Et2O)2[2.4]..................................................................50Scheme 3.2 Synthesis of Brønsted acid H(DMA)[3.1]................................................................. 71Scheme 3.2 Synthesis of Brønsted acid H(aniline)[3.1]................................................................74Scheme 3.3 Synthesis of Brønsted acid H(DMA)[3.2]................................................................. 76Scheme 3.4 Synthesis of Brønsted acid H(aniline)[3.2]................................................................78xvList of Abbreviations alphaÅ Angstrom (1 x 10–10 meters)anal. analysisAr arylavg. averageax axial betabr broad or broadened (spectra)BR butyl rubbernBu n-butyltBu tert-butylC Celsiusc centi (10–2)ca. circa (about)calcd calculatedcis same sideCCDC Cambridge Crystallographic Data CenterCOSY correlation spectroscopyCp cyclopentadienyl ligandxviCp* pentamethylcyclopentadienyl ligandΔ delta (configurational)° degree (angle or temperature)δ NMR chemical shift in parts per million (ppm)d day(s); doublet (NMR spectroscopy)d deuteratedD dimensionalD1 relaxation delay (NMR spectroscopy)d7 inversion recovery delay (NMR spectroscopy)DMFDMAdimethylformamide, (CH3)2NCHdimethylaniline, Ph-N(CH3)2Đ dispersitydn/dc refractive index incremented. editione.g. exempli gratia (for example)elem. elementalESI electrospray ionizationeq equatorialequiv equivalentEt ethyl or CH3CH2–xviiet al. and othersEt2O diethyl ether or (C2H5)2Oetc. and so forthfac facial (configuration)FT Fourier Transformg gramGPC gel permeation chromatographyh hour{1H} proton decoupled (NMR spectroscopy)HPLC high-performance liquid chromatographyHMBC heteronuclear multiple bond correlationHRMS high resolution mass spectrometryHz Hertz (s–1)i iso (as in i-Pr)i.e. id est (in other words)int internal (X-ray)in situ in place or in the reactionin vacuo in a vacuumxviiiIR infraredJ coupling constant (NMR spectroscopy)K KelvinK spectral lineki rate constant of initiationkp rate constant of propagationktr rate constant of terminationKH potassium hydride wavelengthΛ lambda (configuration)L generic ligand; literLA Lewis acidLLS laser light scatteringLRMS low resolution mass spectrometryv wavenumberM generic metal; molarity (mol per liter)M+ molecular ionm milli (10–3); multiplet (NMR spectroscopy); meter micro (10–6); absorption coefficient (X-ray)MAO methylaluminoxane, (–[O–Al(CH3)]n–)MALDI-TOF matrix-assisted laser desorption ionization - time of flightMALS multi angle light scatteringxixMe methyl or CH3–MeOH methanolMeOSt methoxystyrenemer meridonial (configuration)MHz megahertzM/I monomer to initiator ratiomin minuteMn number average molecular weightMw weight average molecular weightMW molecular weightmol moleMS mass spectrometrym/z mass-to-charge ration number; normal (in n-butyl); nanoNA not availableNMR nuclear magnetic resonanceNOESY nuclear Overhauser effect spectroscopyo orthop para% percent (parts per hundred)xxPh phenylpH negative logarithm of hydrogen ion concentrationPhen phenanthrylPhNH2 aniline, C6H5NH2PhNMe2 N,N-dimethylaniline, C6H5N(CH3)2pKa pK for associationppm parts per millionpy pyridineiPr isopropylq quartetR generic substituent; residual factor (X-ray)R rectus (configuration)rac racemic mixture or racemateRh hydrodynamic radiusROESY rotating-frame overhauser effect spectroscopyRT room temperaturervdw van der Waals radiis second; singlet (spectra); strong (spectra)SADABS Siemens area detector absorption correction programxxit tertiary; triplet (NMR spectroscopy); timeT temperatureTc ceiling temperatureTg glass transition temperatureTHF tetrahydrofuran, C4H8OTm melting temperaturetrans opposite sidetrifyl trifluoromethanesulfonylUBC University of British ColumbiaV volume vibrationvide infra see belowvol% volume percentw weak (spectra)WCA weakly coordinating anionwt % weight percentvw very weak (spectra)X generic halogenZ number of units in a cell (X-ray)xxiiAcknowledgmentsFirstly, I will thank my professor, Derek Gates. I appreciate that Prof Gates gave me achance to join the group and learn polymer related material. Professor Gates mentored me in thearea of research with suggestions and improvements when required. I appreciate your patience,kindness, support and all the help you gave me. I had challenges with my health and research,and I wouldn’t have made progress without your help.Secondly, I would like to thank my group mentor, Khatera Hazin. I cherish the years youspent with me. You gave me so many good memories in our lab. I remember every moment Ilearned from you at your fume hood. I had little experience with polymer experiments before youstarted teaching me. I believe you are the most significant mentor I’ve had because you gave mea lot of guidance in and out of the lab.Thirdly, I would like to thank Henry Walsgrove, Zeyu Han, Jeffery Suen, Tian Zhang,and Tanvi Garg. I appreciate your patience in answering my questions, teaching me about severaloperation skills, and discussing various topics.I also want to thank my fellow group memberswhom I co-worked with these past two years. You all made the two years in the lab enjoyableand memorable. Furthermore, I want to thank the NMR spectra, Mass Spec, X-ray Lab,Chemistry Shop, Mechanical Shop, Glass Shop and others for all the work you did. Dr. Brian. O.Patrick, I appreciate your work on collecting and refining crystal data and your patience with mewhile taking your course.I will also thank my mom, she gave support me a lot both morally and mentally. Lastly, Iwant to thank my friends, who made my life more enjoyable, including my dear bunny, Oreo.She kept me company for nearly one year and gave me a lot of comfort and support.1Chapter 1: Introduction1.1 Rubber HistoryHumans have been using rubber for more than 1000 years. In ancient times, rubber cameentirely from nature, specifically from hevea brasiliensis, the rubber tree.3 The white liquid,rubber latex, is stored at vacuoles, after cutting the rubber tree, the latex saps from the wounds.With further treatment and refinement, the natural rubber possesses many desirable properties,such as tear resistance resilience, elongation and high tensile strength.1,2 Therefore, naturalrubber gradually gained wide applications, and the commercial demand for rubber increasedrapidly.Rubber consumption increased dramatically as technology improved in the latenineteenth century.4 Some locations near the Amazon, for example, Manaus and Brazil,progressively became the primary sources of natural rubber worldwide. However, becauserubber trees can only be cultivated in particular climate in tropical rain forests, natural rubberoutput was restricted, potentially impeding the industrial revolution.5 Fortunately, syntheticrubber began to develop and spread during this time period, which may gradually replace naturalrubber. As technology advanced, the yield of natural and synthetic rubber both rose rapidly tofulfill the severe rubber demand.6 In 1855, only 2,100 tonnes of natural rubber were shippedfrom the Amazon. However, in 2020, the global natural rubber output was 12 million tonnes,whereas the global synthetic rubber output was approximately 15 million tonnes.25,26 However,the production of natural rubber has posed serious environmental concerns, particularly tobiodiversity and deforestation, including air, water, and odor pollution.27 These unprecedentedchallenges are especially acute in southeast Asia, which supplies more than 90% of the world's2natural rubber supply. To alleviate environmental pressures, increasing synthetic rubberproduction and eventually replacing natural rubber should be considered.Figure 1.1 The structure of natural rubber’s main components, cis-polyisoprene.Synthetic rubber began its story after “the Macromolecules Hypothesis" was proposed byStaudinger in the 1920s.7-8 In the following years, many synthetic routes to rubber weredeveloped by academic and industrial researchers, which led to the establishment of many newcompanies to manufacture and supply synthetic rubber. In the 20th century, due to thedevelopment of synthetic rubber, finding a location with a suitable climate for growing therubber tree was no longer the primary controlling factor for the global supply of rubber products.Instead of the Amazon and Southeast Asia, advanced industrial countries such as the USSR andGermany gradually became the world's top rubber manufacturers.9-11 Gradually, synthetic rubberhas been used in various products, including vehicle tires, plastic gloves, pencil erasers,swimming caps, and waterproof outerwear. Synthetic rubber, along with synthetic fiber andsynthetic plastic, has become irreplaceable parts of our daily lives.12–14Later in World War II, many outstanding scientists in the organic, physical, and surfacechemical areas made significant contributions to synthetic rubbers.15–17 Specific importantconcerns emerged as the investigation progressed. To modify the properties of synthetic rubber,industry had to perform polymerization reactions at extreme conditions; for instance, thetemperature had to be kept below -70°C.18,19 As a consequence of the continuous efforts byresearchers several solutions to these challenges have been created. Firstly, various chain transfer3agents, such as thiols and disulphides, have been developed to resolve these problems.20,21Secondly, various prospective initiators have been developed in order to used in a mild syntheticenvironment.22-24This thesis presents the development of new Brønsted acids as prospective initiators incationic polymerization to synthesize butyl rubber at moderate temperatures. These novelBrønsted acids will be characterized and investigated in an effort to understand their structuresand their potential use in polymerization. Hopefully, all of these efforts will contribute to thefuture development of high-performance industrial cationic initiators in the longer term.1.2 Cationic Polymerization1.2.1 Cationic Polymerization General MechanismPolymers are long-chain compounds formed by the covalent bonding of multiplerepeating small units known as monomers. Polyethylene (PE), polyvinyl chloride (PVC),polypropylene (PP), and polystyrene (PS) are all common polymers found in everyday life.28There are several polymerization mechanisms, three of which are particularly significant. Theyare radical polymerization with radicals acting as initiators, anionic polymerization withnucleophiles acting as initiators, and cationic polymerization with cations acting as initiators.This section will emphasize the cationic olefin polymerization mechanism because this thesisfocuses on butyl rubber, which can only be generated through cationic polymerization.29 Cationicpolymerization mainly includes three steps: initiation to start polymerization reactions,propagation to grow the polymer chains, and termination to end the polymerization reactions.30The initiation step of cationic polymerization requires cationic initiators, which usuallyare highly reactive carbocations or protons.31 The reaction begins with the electron attacking. As4shown in Scheme 1.1, the nucleophilic olefin portion of the first monomer will attack thecationic initiator, X+. The X+ adds reguiselective to the less hindered side of the monomer toafford the substituted carbocation, X-CH2-CRH+. Traditionally, the X+ component of cationicinitiator systems is H+ and is derived from common protic acids, such as HClO4, CF3COOH, andH2SO4.32–34 However, these protic acids only produce low molecular weight polymers since theanions, A–, in these protic acids coordinate strongly to the active cations and, consequently, tendto quickly terminate the growing carbocations. Recently, more well designed Brønsted acids,which can produce better stability of the carbocationic intermediate, have been utilized incationic polymerization to generate high molecular weight polymers.35 Thus, this thesis will alsotake advantage of Brønsted acids to manufacture specific effective cationic initiators.Scheme 1.1 General mechanism of cationic chain polymerization.5After the first carbocation is formed, the nucleophilic olefin portion of the secondmonomer will attack the carbocation in the same way. Then, other monomers will repeat thisprocedure to form a long-chain molecule. The attachment of monomers is known as propagation,which turns monomers into high molecular macromolecules.36 Undesirable reactions may occurduring propagation, like hydride shift rearrangements.37 And the rearranged products are majorside products when rearrangement rate is fast, while the random co-polymers become theprimary side products when the rearrangement rate is low.Termination may happen in different conditions. First, the growing polymers maycombine with a counterion and terminate the reaction by forming a neutral molecule. Second, thegrowing polymers may also react with impurities or solvent since the propagating species ishighly reactive.38 Furthermore, many other chain transfer reactions may happen to terminate thepolymerization. For example, protons from growing polymer may transfer to monomers or tocounterions, hydride ions may transfer from monomer, and intermolecular hydride ions maytransfer to another polymer.39 These chain transfer reactions will result in low molecular weight,significant dispersity, low purity and low yields.Understanding the mechanism and properties of cationic polymerization is necessary toprevent unwanted side reactions, termination or chain transfer reactions. So the next chapter willdiscuss cationic polymerization and offer some design suggestions for cationic initiators.1.2.2 Properties and Influential Factors of Cationic PolymerizationAmong the three types of polymerization, cationic polymerization is one or more ordersof magnitude faster than anionic and radical polymerization.43 Therefore, cationic, anionic, andradical polymerization reactions take seconds, minutes, and hours to complete, respectively.446Because of the rapid reaction rate, more chain transfer reactions and other side reactions willoccur, making cationic polymerization harder to control. And the termination and chain transferreactions result in polymers with lower molecular weight and greater polydispersity index (PDI).Thus, all the influencing factors should be carefully considered before designing initiators andconducting reactions.Temperature, solvent type, monomer type, and counterions are the most important factorsaffecting cationic polymerization. Kinetically, lowering the temperature reduces the reaction rate,thus, decreases the termination and chain transfer probability simultaneously.45Figure 1.2 Common types of propagating species observed in cationic polymerization.Likewise, the nature of the ions and also the solvent can also influence the type ofpropagating species observed in a cationic polymerization and, consequently, the rate ofpropagation. When cationic initiators are dissolved in solvents, the cations and anions dissociate.The polarity of the solvent and the donor ability of the anion heavily influences the reactivity ofthe cationic propagating species towards monomer. Figure1.2 summarizes the four differenttypes of ion contacts. For strongly donating anions and in non-polar solvents, the cations andanions can show a high degree of covalency, as illustrated in Figure 1.2a. In contrast, moreweakly coordinating anions can lead to ion contacts ranging from close ion pairs (Figure 1.2,b)to solvent separated ion pairs (Figure 1.2,c) to free ions (Figure 1.2,d).47 Typically, more polarsolvents will favour the formation of free ions, thereby increasing the reactivity of thepropagating carbocation. However, polar solvents can also function as donor species and can7bind to the propagating carbocation to reduce its reactivity. Thus, a delicate balance must bestruck in terms of anion choice and solvent choice to favour polymerization.The type of connection between cations and anions affects all three stages of cationicpolymerization: initiation, propagation, and termination.46 Covalently bound pairs (Figure 1.3a)have no functional cations to react with monomers and, as a result, this type of propagatingspecies does not generally give high polymers. Non-polar solvents such as pentane, hexane, andbenzene are hence ineffective in cationic polymerization. Free ions with weakconnections between cations and anions (Figure 1.3.d) can cause early termination and traintransfer processes, creating polymer with low molecular weight and high PDI.48 Thus, highlypolarity solvents, such as water and alcohol, are likewise unsuitable for cationic polymerization.As a result, slightly polar aprotic solvents are the ideal choices since they can create close ionpairs and solvent-separated ion pairs. Solvents such as methylene chloride and 1,2-dichloroethane, for example, work well in cationic polymerization.49Similarly, the size and charge of the counterion will also impact the strength of theinteraction between cations and anions. The electrostatic force in cationic initiators is anattractive force between cations and anions, and the force strength mainly depends on theaverage charge of these ions. As a result, the counterion can considerably impact propagationrates along with termination or chain transfer rates. In general, a smaller charge and larger sizecounterion can lower the electron density by delocalizing electrons to entire anion. Therefore,larger counterions with low overall charge are preferred in cationic initiator design.5081.3 Initiators Used in Cationic PolymerizationCationic polymerization has been utilized to polymerize different monomers from thetwentieth century.30 Numerous types of initiators have been produced. Generally, cationicpolymerization initiators are classified into two types: Lewis acids and Brønsted acids. Lewisacid initiators are binary initiator systems that typically contain Lewis acids and proton donors,such as the AlCl3/H2O system. Brønsted acids initiators are single component initiators thatonly include a single chemical. Furthermore, the properties and structures of Lewis acid andBrønsted acid initiators differ significantly.1.3.1 Binary InitiatorsKennedy is a pioneer in binary initiators. He has established many cationic binaryinitiator systems and investigated their polymerization properties.51 The most significant binaryinitiator Kennedy proposed was cationic initiators containing Et2AlCl with traces of water orHCl. The Et2AlCl binary initiator was frequently used for polymerizing isobutylene (IB).52 Dueto the commercial importance of binary initiators, many modified binary initiators thatperformed well in cationic polymerization have been developed. For example, I2/HI system andBCl3/cumyl acetate were two remarkable breakthroughs for living cationic polymerizationreactions.53,54 It was because BCl3 systems could considerably limit chain transfer whenpolymerizing isobutylene(IB).55 Subsequently, more and more cationic initiators for differentmonomers won great attention. For example, zinc acetates, zinc halides, and SnBr4 performedwell when conjugated with hydrogen halides or acetic acids.56,57 Besides, B(C6F5)3 and itsderivatives have also proven to be good cationic initiators because of the water-tolerantcharacteristics and Lewis acidity.58 For example, (η5-C5(CH3)5)Ti(CH3)3/B(C6F5)3 and Cp*TiR3/9B(C6F5)3, [Ph3C][B(C6F5)4/(C5Me5)Sc(BH4)2(THF)] were all excellent binary initiator systems.59–61Figure 1.3 Mechanism of cationic polymerization of isobutylene using a binary initiator system.However, the initiating species of binary initiators were not precisely confirmed due totheir complexity. Therefore, the mechanism of cationic polymerization using binary initiators ishard to comprehend and investigate. Furthermore, the binary initiators were difficult to controlthe monomer-to-initiator ratios. Thus, researchers have considered constructing single-component initiators which avoid the disadvantages of binary systems. As a result, Brønstedacids gradually gained more attention in cationic polymerization field.101.3.2 Single-Component Initiators for Cationic PolymerizationUnlike binary initiators, some single component initiators can be isolated and in somecases they are isolable as solids. These single-component systems address many of thedisadvantages of binary initiators. For instance, the exact identity of the cationic groups and themechanism of initiation for isolable single component initiators may be easier to elucidate.Moreover, the fact that some single-component initiators are often weighable as solids enablesresearchers to accurately control the monomer-to-initiator ratios. However, single-componentinitiators are less explored than binary initiators, since the synthesis, isolation, and storage ofthese Brønsted acids are more complicated than Lewis acids used in binary initiator systems.62Single-component initiators were often extremely sensitive to air and moisture and must bestored at low temperatures.Given the potential advantages of solid Brønsted acids, the development and study ofnew types of single-component initiators clearly merits additional effort. A key to thesedevelopments will involve the design of sophisticated counterions to create a more robuststructure for Brønsted acid initiators. Research has shown that the weakly coordinating anions(WCAs) may be the suitable counterions for high activity initiator systems due to its moderatecoordination nature.63 Thus, the next chapter will briefly discuss the properties, history anddifferent weakly coordinating anions (WCAs).1.4 Weakly Coordinating Anions (WCAs)Weakly Coordinating Anions (WCAs) are anions that can coordinate target cations withweak interaction. Rosenthal first proposed the term “Non-Coordinating Anions” to describeanions that are of very low reactivity and do not coordinate to cation.68 Early examples included11ClO4-, NO3- and BF4-. In the later 1990s, the more accurate term, “Weakly Coordination Anions,”gradually replaced the “Non-Coordinating Anions” since researchers have shown that it is notreally possible to completely prevent weak bonding interactions between a cation and an anion.69Scheme 1.2 Three different interactions between the cationic parts and anionic part, and the sideeffects of the other interactions for cationic polymerization.The coordinating nature of anions can affect the reactivity and stability of cations due totheir electrostatic attraction to cations. This is of particular importance in cationic polymerizationas shown in Scheme1.2. If too strong an interaction is present, the anion will bind to the cationand will not permit propagation (i.e. termination will occur). If there is too weak an interaction,the cation will not be protected (i.e. it is too reactive) and side reactions such as chain transfercan occur in competition with propagation. The ideal interaction between a propagating cationand anion is optimal to favor propagation over undesirable termination and chain transferreactions. Reaching this optimal balance of cation-anion interaction is highly challenging.Weakly coordinating anions (WCAs) with electron withdrawing ligands and a largevolume to delocalize the negative charge are most desirable for polymerization applications. Asshown in Figure 1.4,2, the negative charge of weakly coordinating anions (WCAs) is dispersed12within the anion rather than being bound to a single site, resulting in a weakly coordinatingnature. In contrast, small anions typically bind to cations more strongly due to the localization ofthe negative charge onto a small number of atoms (Figure 1.4,1). Thus, WCAs performed well instabilizing cationic metal complexes initiators for cationic polymerization.63Figure 1.4 1). Interaction between cations and small, nucleophilic anions 2). Interaction between cations andweakly coordinating anions (WCAs).Understanding the evolutionary history of WCAs will assist in studying the designprinciples of using WCAs in stable cationic polymerization initiators. As a result, the followingsections will introduce research involving cationic initiators that are comprised of weaklycoordinating anions (WCAs). To date, WCAs featuring p-block elements being have been mostattractive with those comprised of Groups 13 and 14 elements bound by large electron-withdrawing ligands being well-established.73 Although receiving less attention than the maingroup elements, transition metals have recently provided some potential for the design of WCAs.The following sections will go over different WCAs with different central elements.1.4.1 B-centered WCAsB-centered WCAs were first used to replace the traditional anion, BF4-, with the largeranions, BPh4-, to form initiators which postulated to performed well in polymerization.7413However, it was discovered that initiators with BPh4- counterion were significantly less active forethylene polymerization than catalysts with [BF4]-.75 This was due to the π-coordination bondsexisting in the phenyl groups of BPh4- , which was indicated by x-ray crystallography.76Later, researchers proposed several methods for improving the performance of [BPh4]-anions. Researchers investigated anions bearing fluorinated aryl groups, such as B(C6F5)4- and[B(3,5-C6H3(CF3)2)4]- (Figure 1.4).77 It is because the electron-withdrawing fluorine elementmight greatly delocalize the negative charge in the phenyl group, and make the whole WCAslarger at the same time. Therefore, the closest contacts between the outermost atoms of theseWCAs and the cations were beyond the van der Waals radius.78 These fluoroaryl-substituted B-centred anions showed improved stability compared to their unfluorinated analogues, and weremore resistant to B-C bond cleavage.79,80 As a result, F-containing derivatives of B-centeredWCAs represented a milestone in WCAs development. These fluoroaryl-substituted B-centredWCAs have been applied as coordination polymerization initiator systems for olefinpolymerization and species, such as [Zr(Cp*)2(CH3)][B(C6F5)4] and[Th(Cp*)2(CH3)][B(C6F5)4].85,86As discussed earlier, larger anions usually perform better as cationic initiators.Introducing nitrogen or oxygen elements to the B-containing WCAs represents another approachto constructing larger anions. Those larger groups were synthesized by bridging two small boroncomplexes with nitrogen and oxygen elements. Thus, the entire WCAs will contain more atomsand the negative charges of those anions may consequently be be delocalized to a larger volume.For example, [Ph3BCNBPh3]- represents a new functional group for crystallization andstabilization of the cationic group, [Ph3P=N=PPh3]+.87 Even though the anion is larger than theBPh4-, [Ph3BCNBPh3]- still showed and its undesirable π-coordination was almost as strong as14it in BPh4-. Thus, F-derivatives were sought to address this issue.88 As shown in Figure1.5,[B(C6F5)3-N-B(C6F5)3]-, [B(C6F5)3-CN-B(C6F5)3]-, [B(C6F5)3-O-CR-O-B(C6F5)3]- were allexcellent WCAs with multiple boron centers.89–92Figure 1.5 Structures of B-centered WCAs.Carborane anions, represent another significant class of WCAs based on boron element.Similarly, halogen derivatives were sought for their lower reactivity than fully H- or C-substituted carboranes. Perhaps the most prominent class are [CB11R6X6]– (R = H, CH3, X = F,Cl, Br), which have shown resistance to oxidation and acid cleavage compared to carboraneswithout halogens.87,89151.4.2 Al-centered WCAsThe success of B-containing WCAs had inspired the development of WCAs featuringheavier group 13 elements (Figure 1.5). For instance, Marks and his colleagues designed an Al-centered WCAs, [AlF(C6F4(C6F5))3]-, that is similar to the traditional B-centered WCAs.93However, the molecule [Ph3C][AlF(C6F4(C6F5))3] did not function as an cationic initiator.Instead, it was employed as a co-catalyst for Zr-centered initiators in polymerizing ethylene andpropene.94,95 Some comparative experiments indicated that [AlF(C6F4(C6F5))3]- might havestronger coordinating strength than the similar B-centered WCAs. Meanwhile, the performanceof [AlF(C6F4(C6F5))3]- in olefin polymerization reactions was not as effective as B-centeredWCAs.96Researchers borrowed similar design ideas from B-centered WCAs to solve thecoordinating problems of Al-centered WCAs. [AlF(C6F5)3]- was modified by bridging severalaluminum centers with fluorine, oxygen and many other elements. This operation would increasethe volume of Al-centered WCAs. Specifically, two or three aluminum centers were connectedwith fluorine and created some functional WCAs, like [F(Al(C6F5)3)2]-, [F2(Al(C6F5)3)3]2-, asshown in Figure 1.5.97,98 Their larger size was expected to separate the negative charge anddecrease the coordinating strength. Marks and his colleagues found that the increased size ofWCAs trend did not always improve their activity in the zirconocene-catalyzed polymerizationof propene. Instead, [F(Al(C6F5)3)2]- showed the best reactivity among all the first four Al-centered WCAs shown in Figure 1.6.97,98 Changing the bridging element is another directionemployed to increase the size of Al-centered WCAs. Many researchers showed that addingoxygen elements between the Al-C bonds results in dramatically improved performance, such as16[Al(OC6F5)4]– and [Al{OC(CF3)3}4]–.99,100 Moreover, these anions are typically easier tosynthesize.Figure 1.6 Structure of Al-centered WCAs.1.4.3 Other Main Group Elements Centered WCAsExcept for boron and aluminum, many other elements from the p-block have been alsoapplied as central atoms for WCAs. For example, gallium, sulfur, antimony, tellurium all leftglorious marks on the history of WCAs.(references) Some Gallium-containing WCAs stillperformed well as initiators for the cationic polymerization, such as [Ga(C6F5)4]- and [(C6F5)3Ga-F-Ga(C6F5)3]-(Figure 1.7).101,102 Meanwhile, described as highly charge-delocalized anions, the S-17contained anions also worked well as WCAs, such as [SO2(CF3)-N-SO2(CF3)]- and[C{SO2(CF3)}3]-.103,104 Sb-centered WCAs were synthesized from the 1960s. Those WCAsdiscovered in the early period also deserved to be mentioned, like the groundwork of [F2(SbF5)3]-and [F3(SbF5)4]-.105,106 Besides, [OTeF5]- was a suitable ligand for WCAs because of its bulkinessand electronegativity.107 The significant advantage of [OTeF5]- is that it could increase similargroups' solubility and change compounds' crystal structures due to the strong M-O bond.110Moreover, when [OTeF5]- work as WCAs, the whole compounds tend to form dimer instead ofmonomer.111 Besides, other non-metal and metal elements were also used in creating largerWCAs, such as arsenic, antimony, bismuth, tantalum, niobium.108,10918Figure 1.7 Other WCAs containing p-block elements.1.5 Brønsted Acids with WCAsBrønsted acids featuring the types of WCAs described above are promising initiators forcationic polymerization reactions. The cation, HLx+, is a proton surrounded with several weaklybasic solvent molecules, L, which is the structure for Brønsted acids with WCAs.59 Figure 1.8showed two oxonium cations, [H(Et2O)2]+, with Al-centered and B-centered WCAs. The twoBrønsted acids with WCAs were effective single component initiators for the ring-opening19polymerization of 2-alkyl-2-oxazolines.69, 76These Brønsted acids with WCAs provided thepossibility for design cationic initiator for rubber.The Gates group has designed many Brønsted acids, HL2+(L = Et2O, THF, DMF, DMSO,and many other solutions) with P-centered WCAs. Moreover, transition metals from group 5,such as tantalum and niobium, have been employed as the central atoms of WCAs to stabilize theBrønsted acids, HL2+(L = Et2O, THF, DMF, DMSO, and many other solutions). These cationicinitiators were all weighable solids and effective as single-component initiators for cationicpolymerization of olefins. The fact that they are solids makes them particularly desirable asinitiators since they can be accurately weighed on a balance to give very accurate monomer-to-initiator ratios. Besides, these initiators performed well in polymerizing many different olefinmonomers. This thesis will focus more on initiators with Ta(V)- or Nb(V)-containing WCAs.Figure 1.8 The two single-component initiators applied in cationic polymerization.1.6 Outline of the ThesisThis thesis focuses on synthesizing and characterizing the H(L)2[M(1,2-O2C6Cl4)3] andH(L)2[M(1,2-O2C6Cl4)2(1,2-O(OH)C6Cl4)2] (M = Ta, Nb) systems with a variety of ligandssolvents (L). By using solvents containing nitrogen like dimethylformamide, dimethylaniline andaniline, more stable H(L)2[M(1,2-O2C6Cl4)3] and H(L)2[M(1,2-O2C6Cl4)2(1,2O(OH)C6Cl4)2]20(M=P, Ta, Nb) Brønsted acids were synthesized. Moreover, two novel heptacoordinated WCAs,H(DMF)2[Ta(1,2-O2C6Cl4)2(1,2–O2(OH)2C6Cl4)2O=CHN(CH3)2] and [H(DMA)]2[Ta(1,2-O2C6Cl4)3(1,2–O2(OH)2C6Cl4)1] were discovered. The two heptacoordinated WCAs wereunprecedented structures and different from those WCAs analyzed earlier. Chapter 2 summarizesand compares the Brønsted acids with Ta-containing WCAs. Seven cationic initiators withdifferent ligand ratios and basic solutions were synthesized and characterized. Chapter 3examines four niobium(V) analogs with similar structures. Although niobium and tantalumbelong to the same group of the periodic table, they show some different features. Chapter 4concludes all the work in this thesis and will propose some possible modification methods forfuture work.21Chapter 2: Single Component Cationic Initiators with Hexacoordinate andHeptacoordinate Ta(V)-containing Weakly coordinating Anions2.1 IntroductionAs illustrated in Scheme 2.1, a Brønsted acid HA can dissociate in solution to liberatesolvated protons (H+) and counterions (A- ). Since the cationic parts only contain protons, H+,Brønsted acids are promising single component initiators for olefin polymerization. The contactintensity between cations and anions in solution influences the activity of Brønsted acids ascationic initiators. Research has shown that weakly coordinating anions (WCAs) are promisinganions to generate highly reactive Brønsted acids for application as single-component cationicinitiators for olefin polymerization.28Scheme 2. 1 Equilibrium of solvated proton and counterions from Brønsted acids.Traditionally, boron and aluminum were widely used in WCAs, however, compared toelements from group 13, elements from Group 15, like P, were less used in WCAs. It wasn't untilthe 1970s that several stable phosphorous anions(V) were synthesized and investigated. Somestudies have identified hexacoordinate phosphorus compounds as WCAs, as illustrated in Figure2.1, such as [PF6]– and [PCl6]– and [P(O2C6H6)3]-.119,120 The tris(tetrachlorobenzenediolato)phosphate drew our groups' attention to developing single component initiators for cationicpolymerization using P-containing WCAs. The procedure to synthesize the P-centered Brønstedacids, H(OEt2)2[P(O2C6Cl6)], was quite straightforward after careful design. It only required PCl5to react with 1,2-C6Cl4(OH)2 (3 equiv) followed by then adding a weakly basic solvent such asEt2O to generate and stabilize the functional cations, H+. Inspired by the success of the22H(OEt2)2[P(O2C6Cl6)], our group members made several Brønsted acids with P-centered WCAs,which are shown in Figure 2.2.121–124Figure 2.1 Structure of three traditional P-centered WCAsFigure 2.2 Structures of P-centered single component initiator from our group.In addition to the hexacoordinated P series of single component initiators, our group hasattempted to access the tantalum(V)–containing WCAs. Tantalum belongs to group 5 whereasphosphorus belongs to group 15, and elements in both groups form compounds where theelement adopts a +5 oxidation state. Figure 2.3 gives the structural formula of several Brønstedacids featuring Ta-containing WCAs. Previous studies revealed that the Ta(V) center of the23WCA in these acids has a coordination number of 6. Thus, a large number of big ligands cancoordinate to tantalum(V).125–127 Ta(V)-containing anions are often large enough to exhibit weakto moderate coordinating strength for cations which are promising to stabilize Brønsted acids.[Ta(OC2H5)(C2O4)3]2- was a WCA with a -2 overall charge with distorted pentagonal bipyramidgeometry.126 Our group also created two types of single component initiators for olefinpolymerization using Ta-containing WCAs, which are shown in Figure 2.4.128Figure 2.3 Structures of traditional Ta-centered WCAs.24Figure 2.4 Structures of Ta-centered single component initiator from our group.When WCAs are employed as anions, the cationic part, H+, normally require a weaklybasic solvent, L, for stabilization. Hence the entire cationic component is indicated as HL2+.115Moreover, the basicity of the solvent L may affect the stability and the ability of isolation ofBrønsted acid salt. Therefore, it was hypothesized that employing stronger basic solventscontaining nitrogen element, like dimethylformamide, dimethylaniline and aniline, might affordBrønsted acids with more robust structures. Specifically, initiators with 1:4 ratio Ta(V) WCAs,HL2[Ta(V)(1,2-O2C6Cl4)2(1,2-O(OH)C6Cl4)2] (L=DMF, DMA, aniline, THF, Et2O), and 1:4 ratioTa(V) WCAs HL2[Ta(V)(1,2-O2C6Cl4)3] (L=DMA, THF, Et2O) were synthesized andcharacterized herein.Among all the synthesized initiators, the two heptacoordinated Ta(V) WCAs were themost intriguing ones, since their structures are completely different from the initiators our labpreviously synthesized. As a result, these two Ta-centered WCAs will be the main topic of thischapter. Besides, various cationic initiators with Ta-centered WCAs will also be covered in thischapter.252.2 Results and Discussion about cationic initiators with Ta-centered WCAs2.2.1 [H(DMF)2][2.1]Our group have isolated Brønsted acid salts HL2[Ta(1,2-O2C6Cl4)3] (L= Et2O) andHL2[Ta(1,2-O2C6Cl4)2(1,2-O(OH)C6Cl4)2] (L= Et2O). The procedure was straightforward.Tetrachlorocatechol was reacted with TaCl5 in CH2Cl2 solution to afford some functional WCAs.After adding Et2O as the basic solution, some efficient Brønsted acids formed. These solid andweighable compounds were well-performed single-component initiators for olefinpolymerization. Specifically, they successfully polymerized a wide range of monomers, likevinyl ether, styrene, methoxy styrene, and isoprene. Importantly, they were also effective inmaking butyl rubber.113 The characterization results revealed that the two large WCAs hadhexacoordinate structures.DMF was used instead of Et2O with the goal of forming a more stable Brønsted acid withthe Ta(V) WCA coordinated with tetrachlorocatechol ligands. In a glovebox, two Schlenk flaskswere charged with tetrachlorocatechol and TaCl5 (4:1 ratio), respectively. The flasks weretransferred to a Schlenk line and placed under a flow of nitrogen. To each was added CH2Cl2 (ca.3 mL each) and the resultant mixtures were stirred and heated by immersion in a 90 °C oil bathuntil the solid reactants dissolved. The orange tetrachlorocatechol solution was then added to thecolourless solution of TaCl5. The Schlenk flask containing the reaction mixture was equippedwith a water-cooled condenser and immersed in an oil bath heated to 90 °C for around one hour.The resultant solution was cooled to 20 °C and 0.5 mL of DMF was added. The crystals wereanalyzed using X-ray crystallography and were identified as H(DMF)2[Ta(1,2-O2C6Cl4)2(1,2–O2(OH)2C6Cl4)2(O=CHN(CH3)2)]. Scheme 2.2 depicts the full reaction route.26Scheme 2.2 Procedures for synthesizing Brønsted acid H(DMF)2[2.1].The crystals were dissolved in CDCl3 solution for 1H NMR spectroscopic characterizationat ambient. The room temperature 1H NMR spectrum of H(DMF)2[2.1] in CDCl3 showed thesignal of three DMF molecules (δ =3.03, 18H, N(CH3)2; δ =8.12, 3H, O=CH) (Figure 2.5).Meanwhile, the low-temperature NMR spectrum showed similar signals that suggested that twotypes of DMF molecules were present with 2:1 ratio (2 DMF: δ =2.83, 12H, N(CH3)2; δ =7.99,2H, O=CH; 1 DMF : δ =3.07, 6H, N(CH3)2; δ =8.14, 1H, O=CH) (Figure 2.6). This may indicatethat two DMF molecules existed in the cation (HL2+) as the ligands L, and one DMF moleculeexisted in the WCA as a ligand for the seven coordinated tantalum center, which was confirmedby the X-ray crystallography.27Figure 2.5 Room temperature 1H NMR (400 MHz, CDCl3) spectrum of [H(DMF)][2.1].The room temperature 1H NMR spectrum displayed no signal around 19 ppm. However, thelow temperature 1H NMR spectrum showed a resonance at at 19.41 ppm as expected for theacidic proton of [H(DMF)2]+ (T = –85 °C, CD2Cl2: δ = 19.41) (Figure 2.6), which was assignedto the active proton. The resonance at 19.41 ppm was similar to the acidic proton resonance ofcationic initiators reported previously. Our group member, Khatera Hazin, has synthesized andfully characterized two Brønsted acids with P-center WCAs. The basic solution she applied wasdiethyl ether, and the proton resonance of H(DMF)2[P(1,2-O2C6Cl4)3] was observed at 19.2ppm.112Besides data from our group, there were data from other experiments involving theproton peaks of related Brønsted acids. The proton peak of the traditional B-centered WCAs,H(OEt2)2[B(C6F5)4], was around 15.5 ppm at room temperature (δ = 15.5, CD2Cl2 at 25 °C).129Another traditional B-containing WCAs, HL2[B(CF3)4](L=Et2O), showed a similar shift for its28proton (δ = 16.3, CD2Cl2 at 25 °C). 130,131 When it came to carborane anions, the proton shiftwere around 12 ppm, for example, H(OEt2)2[CHB11Me5Cl6] (δ = 13.8, C6D6, at 25 °C) andH(OEt2)2[CHB11Me5Br6] (δ = 11.7, C6D6, at 25 °C).132 Brønsted acids with Al-containing and P-containing WCAs also showed similar proton shifts, such as H(OEt2)2[Al{OC(CF3)3}4] (δ = 14.7,C6D6, at 25 °C) and H(OEt2)2[P(C2O4)3] (δ = 15.5, CDCl3, at 25 °C).133,134 When it came to someN-containing WCAs, some research has shown that the proton signals were all close to 16.5ppm.135,136Figure 2.6 Low temperature (-85 °C) 1H NMR (400 MHz, CDCl3) spectrum of [H(DMF)2][2.1].※ indicates impurities. ∷ Indicates grease peak.292.2.2 Crystal Structure Information with Metrical ParametersThe single crystals of Brønsted acid H(DMF)2[2.1] were obtained from a saturated CD2Cl2solution of the crude product. Under an inert atmosphere, the solution formed some light-yellowcrystals within 14 days. The crystal structure exhibits three DMF molecules in the formula unit,two in the cationic part and one in the anionic part. The cation moiety can be concluded to be[H(DMF)2]+. When combined with the weakly coordinating anion and the solvent ofcrystallization, as shown in Figure 2.7, the whole molecular structure has the formulationH(DMF)2[Ta(V)(1,2-O2C6Cl4)2 (1,2–O2(OH)2C6Cl4)2(O=CHN(CH3)2)]·0.7CH2Cl2.Figure 2.7 The single crystal structure of H(DMF)2[Ta(V)-cis-(1,2-O2C6Cl4)2-trans-(1,2–O2(OH)2C6Cl4)2(O=CHN(CH3)2)]·0.7CH2Cl2 is demonstrated. The crystal structure was drawn at 50%probability level of ellipsoids. Solvents of crystallization (0.7 x CH2Cl2) are omitted for clarity. Selected bond30lengths [Å]: Ta(1)–O(1) = 2.152(3); Ta(1)–O(2) = 1.946(3); Ta(1)–O(4) = 2.009(3); Ta(1)–O(5) = 2.041(3);Ta(1)–O(6) = 2.165(3); Ta(1)–O(7) = 2.016(3); Ta(1)–O(8) = 1.964(3);C(9)–O(3) = 1.356(5); C(27)–O(9) =1.341(5); C(15)–O(5) = 1.338(4); C(1)–O(1) = 1.265(5); C(64)–O(22) = 1.238(6);C(61)–O(21) = 1.252(7); O(22)–H(21) = 1.35(9); O(21)–H(21) = 1.22(9); C(1)–N(1) = 1.300(5); C(64)–N(6) = 1.310(6); C(61)–N(5) = 1.309(6).Selected bond angles [°]: O(1)–Ta(1)–O(5) = 68.10(1); O(1)–Ta(1)–O(6) = 72.35(4); O(4)–Ta(1)–O(5) =74.53(10); O(6)–Ta(1)–O(7) = 74.46(10); O(4)–Ta(1)–O(7) = 71.16(9); O(5)–Ta(1)–O(8) = 89.19(11); O(2)–Ta(1)–O(6) = 82.19(11); O(4)–Ta(1)–O(8)= 103.55(11); O(2)–Ta(1)–O(7)= 96.13(11); O(2)–Ta(1)–O(8)=162.23(8); C(62)–N(5)–C(63) = 116.5(4); C(66)–N(6)–C(65) = 117.5(4).The structure of the anion was strikingly different from Dr. Hazin’s initiator containing Ta-centered complex anion, H(OEt2)(H2O)Ta(1,2-O2C6Cl4)2(1,2–O2(OH)2C6Cl4)2. The complexanion in Dr. Hazin’s crystal structure contained two chelated tetrachlorocatechol moieties andtwo monodentate tetrachlorocatechol ligands coordinated in the cis position, which afforded ahexacoordinated tantalum(V) ion. The crystal I synthesized also had the two chelatedtetrachlorocatechol moieties [Ta(1)–O(4) and Ta(1)–O(5); Ta(1)–O(6) and Ta(1)–O(7)] in the cisposition and two monodentate tetrachlorocatechol ligands coordinated in the trans position. But,besides the four tetrachlorocatechol ligands, my crystal had another DMF molecule connected tothe tantalum metal center with [Ta(1)–O(1)] bond. The two chelated tetrachlorocatechol ligands,the two monodentate tetrachlorocatechol ligands, and the DMF molecule led to an overallheptacoordinated tantalum(V) centre in the WCA.Within this novel WCA there were two intramolecular hydrogen bonds between an oxygenof one of the chelating tetrachlorocatecholate and hydrogen atoms of the hydroxyl groups of thetwo monodentate tetrachlorocatechol in WCA [2.1]- [O(9)···H(6) = 1.96(2) Å, O(3)···H(6) =1.99(2) Å ]. These two intramolecular hydrogen bonds were indicated by a dashed line in Figure312.8. Importantly, these hydrogen bonds were different from the those observed in the compoundcharacterized by Dr. Hazin, H(OEt2)(H2O)Ta(1,2-O2C6Cl4)2(1,2–O2(OH)2C6Cl4)2. Specifically,the unbound hydroxyl groups of the two monodentate tetrachlorocatechol ligands were eachconnected to an oxygen atom from one bidentate chelating tetrachlorocatecholates. In addition,in that case the two O···H hydrogen bonds showed a significant difference in length [O(6)···H(4)= 1.91(2) Å; O(2)···H(8) = 2.23(2) Å]. Likely due to the two intramolecular hydrogen bonds andsmaller occupation space of the DMA ligand, the two monodentate ligands moieties were notlinear and bent towards one side, more specifically, towards the DMF ligand side [O(2)–Ta(1)–O(8) = 162.23(11)°], as shown in Figure 2.8.The closest cation-anion contact in H(DMF)2[2.1] occurred between an oxygen atom of theanion [2.1]– and a hydrogen atom of the DMF solvent [O(2)···H(64) = 2.408(2) Å]. This contactwas within the sum of the van der Waals radii for oxygen and hydrogen [rvdw = 2.72 Å].Figure 2.8 The two monodentate C6Cl4O(OH)- ligands moieties are bent towards the DMF molecule.32The bond angles of pentagonal bipyramid geometry were supposed to be 72° and 90°. Thetantalum(V) center in the anion [2.1]- exhibits angles around 72° and 93°, which indicated theH(DMF)2[2.1] had a slightly distorted pentagonal bipyramid geometry. The two monodentatecoordinated tetrachlorocatechol moieties were trans in top and bottom axial positions of thepentagonal bipyramid. However, the O(2)-Ta(1)-O(8) angle of 162.23° deviated from linearitylikely due to steric repulsion or short intermolecular contacts by hydrogen bonds [axial-axial:O(2)–Ta(1)–O(8)= 162.23(8)°; axial-equatorial: O(5)–Ta(1)–O(8) = 89.19(5)°; O(4)–Ta(1)–O(8)= 103.56(4)°; O(7)–Ta(1)–O(8) = 91.52(5)°; O(6)–Ta(1)–O(8) = 84.47(0)°; O(1)–Ta(1)–O(8) =81.32(5)°; O(5)–Ta(1)–O(2) = 93.66(5)°; O(4)–Ta(1)–O(2) = 94.09(4)°; O(7)–Ta(1)–O(2) =96.12(5)°; O(6)–Ta(1)–O(2) = 82.18(0)°; O(1)–Ta(1)–O(2) = 81.83(5)°]. The chelatedtetrachlorocatechol ligands and the DMF molecule occupied positions through the oxygen atoms[Ta(1)–O(4) and Ta(1)–O(5); Ta(1)–O(6) and Ta(1)–O(7); Ta(1)–O(1)] and were slightlyperturbed from regular 72° angles expected in the pentagonal plane [equatorial-equatorial: O(1)–Ta(1)–O(5) = 68.10(1)°; O(1)–Ta(1)–O(6) = 72.35(4)°; O(4)–Ta(1)–O(5) = 74.53(10)°; O(6)–Ta(1)–O(7) = 74.46(10)°; O(4)–Ta(1)–O(7) = 71.16(9)°] (Figure 2.9).The pentagonal bipyramid geometry was quite different from Dr. Hazin’s crystal,H(OEt2)(H2O)Ta(1,2-O2C6Cl4)2(1,2–O2(OH)2C6Cl4)2, which shows distorted octahedralgeometry with two cis coordinated monodentate tetrachlorocatechol moieties [O(3)–Ta(1)–O(7)= 93.0(8)°; O(1)–Ta(1)–O(3) = 95.4(8)°; O(2)–Ta(1)–O(7) = 82.2(7)°; O(5)–Ta(1)–O(7) =86.7(8)°; O(3)–Ta(1)–O(6) = 86.6(7)°]. Besides, [NEt2H2][Ta–(O2C20H10{SiMe3}2–3,3')2Cl2][O–Ta–O, 92.7°; Cl–Ta–Cl, 84.6°], [Ta(OCH3)4{(Me)3CCOCH2CO(Me)3}] [O–Ta–O rangefrom 79.3(2)° to 92.7(3)°] exhibit a pseudo–octahedral geometry, which were also different fromthe pentagonal bipyramid geometry of H(DMF)2[2.1] .33Figure 2.9 Sketch of the center structure of H(DMF)2[2.1].The Ta–O bond distances in H(DMF)2[2.1] ranged from 1.946(3) Å to 2.165(3) Å. Theaverage Ta–O bond lengths in H(DMF)2[2.1] [avg. Ta–O 2.058(34) Å for Ta-O monodentatebonds and 1.955(3) Å for Ta-O bidentate bonds] were slightly longer than Dr. Hazin’s crystal,H(OEt2)(H2O)Ta(1,2-O2C6Cl4)2(1,2–O2(OH)2C6Cl4)2 [avg. Ta–O 2.019(34) Å and 1.906(21) Å].And Ta–O bond lengths of H(DMF)2[2.1] were at the longer end of the typical range found in[Ta(C6H4O2)3py] [avg. 2.025(12) Å], [Ph3C][Ta(OC6F5)6] [avg. 1.952(3) Å], and other tantalum-oxygen-containing bidentate and monodentate systems [avg. range: 2.032(2)–2.16(1) and1.863(3)–1.976(1)].125-127The cation moiety's measured parameters provide valuable insight into the compound'sbonding. It appeared that the acidic proton was coordinated slightly asymmetrically through theoxygen atoms of two DMF groups [O(19)–H(19) = 1.248(4) Å; O(20)–H(19) = 1.117(4) Å](Figure 2.10). The acidic proton was slightly different from the acidic proton of Dr. Hazin’scrystal, H(OEt2)(H2O)Ta(1,2-O2C6Cl4)2(1,2–O2(OH)2C6Cl4)2, which was coordinated34asymmetrically through the oxygen atom of one Et2O and an oxygen atom of one H2O group[O(10)–H(10) = 1.10(4) Å; O(11)–H(10) = 1.50(4) Å].Figure 2.10 Metrical parameters [Å] for the cation in H(DMF)2[Ta-cis-(1,2-O2C6Cl4)2-trans-(1,2–O2(OH)2C6Cl4)2(O=CHN(CH3)2)]·0.7CH2Cl2.2.2.3 [H(DMA)]2[2.2]DMA was used instead of Et2O with the goal of forming a more stable Brønsted acid withthe Ta(V) WCA coordinated with tetrachlorocatechol ligands. In a glovebox, two Schlenk flaskswere charged with tetrachlorocatechol and TaCl5 (4:1 ratio), respectively. The flasks weretransferred to a Schlenk line and placed under a flow of nitrogen. To each was added CH2Cl2 (ca.3 mL each) and the resultant mixtures were stirred and heated by immersion in a 90 °C oil bathuntil the solid reactants dissolved. The orange tetrachlorocatechol solution was then added to thecolourless solution of TaCl5. The Schlenk flask containing the reaction mixture was equippedwith a water-cooled condenser and immersed in an oil bath heated to 90 °C for around one hour.The resultant solution was cooled to 20 °C and 0.5 mL of DMA was added. Upon standing at –30 °C, a light yellow solid precipitated from the solution as yellow crystals after 14 days. Thecrystals were analyzed using X-ray crystallography and were identified as [H(DMA)]2[Ta(1,2-O2C6Cl4)3(1,2–O2(OH)2C6Cl4)1]. The detailed reaction route is shown in Scheme 2.3.35Scheme 2.3 Procedures for synthesizing Brønsted acid [H(DMA)]2[2.2].The product was analyzed by 1H NMR spectroscopy. Importantly, a signal was observed at9.79 ppm which was tentatively assigned to the hydroxyl proton of a monodentatetetrachlorocatechol moiety (Figure 2.11). At low temperature, this hydroxyl proton signal shiftedupfield to 8.06 ppm (T = –85 °C, CD2Cl2: δ = 8.10) (Figure 2.12). Additional support for thisassignment was obtained from the 1H–1H COSY-NMR spectrum which suggested no coupling toother H atoms, consistent with that expected for the hydroxyl moiety of tetrachlorocatechol. Theroom temperature 1H NMR spectrum showed two DMA molecules signal. The resonance at 3.36ppm was assigned to the methyl group of DMA in [H(DMA)]+, and the resonance at 7.26 ppmwas assigned to aromatic protons of DMA in [H(DMA)]+ (δ =3.36, 12H, N(CH3)2; 7.26, 10H,Ph-H). And the low temperature 1H NMR spectrum showed similar results. The resonance at3.41 ppm was assigned to the methyl group of DMA in [H(DMA)]+, and the resonance at 7.04ppm was assigned to aromatic protons of DMA in [H(DMA)]+ (δ =3.41, 12H, N(CH3)2; 7.04,10H, Ph-H).At ambient temperature, the 1H NMR spectrum of [H(DMA)]2[2.2] in CDCl3 also exhibiteda resonance at 10.95 ppm that was assigned to the acidic proton (δ =10.95, 2H, H+). The acidicproton at δ = 10.95 ppm was similar to the acidic proton in previously reported [H(DMA)]-rac-mer-[P-(C6H4CO2)3](δ =9.31 acetone-d6, at 25 °C).121 Interestingly, the low temperature 1H36NMR spectrum exhibited no resonance around 11 ppm, and the active proton signal wasobserved further downfield at 16.79 ppm. This downfield shift was similar to the acidic proton inpreviously reported H(OEt2)2[P(1,2-O2C6Cl4)3] (T = –85 °C, CD2Cl2: δ = 16.70, H),H(OEt2)2[B(C6F5)4] (T = 25 °C, δ = 15.5, CD2Cl2) and H(OEt2)2[Al{OC(CF3)3}4] (δ = 14.7, C6D6,at 25 °C).129,133 When temperature is lowered usually the H shift will shift downfield. Or it maybecause the crystal was set for several days, and the molecular structure may change.Figure 2.11 1H NMR (400 MHz, CD2Cl2, Room Temperature) spectrum of [H(DMA)]2[2.2].※ indicates impurities. ∷ indicates grease peak. ✻ unassigned signal.37Figure 2.12 1H NMR (400 MHz, CD2Cl2, -85 °C) spectrum of [H(DMA)]2[2.2].※ indicates impurities. ∷ indicates grease peak. ✻ unassigned signal.2.2.4 Crystal Structure Information with Metrical ParametersLight yellow crystals of Brønsted acid [H(DMA)]2[2.2] suitable for X-ray crystallographicanalysis were obtained by cooling a saturated solution of the crude product in CDCl3 to –30 °Cunder inert atmosphere for ca. 14 days. The structure of [H(DMA)]2[2.2] was determined as[H(DMA)]2[Ta(1,2–O2C6Cl4)3(1,2–(OH)OC6Cl4)1]·0.5CH2Cl2. The solid state structure exhibitedtwo [H(DMA)]+ cations for every anion, and the WCA, [2.2]2–, had a total charge of –2.Interestingly, the Ta(V) centre in anion [2.2]2- was bound by three chelated tetrachlorocatecholmoieties [Ta(1)–O(1) and Ta(1)–O(2); Ta(1)–O(3) and Ta(1)–O(4); Ta(1)–O(5) and Ta(1)–O(6)]38and only one monodentate tetrachlorocatechol ligands coordinated [Ta(1)–O(7)]. (Figure2.13). .In contrast, Dr. Hazin’s crystal, H(Et2O)2[Ta(1,2-O2C6Cl4)2(1,2–O2(OH)2C6Cl4)2]contained, two chelated tetrachlorocatechol moieties and two monodentate tetrachlorocatecholligands.Figure 2.13 The single crystal structure of [H(DMA)]2[Ta-cis-(1,2-O2C6Cl4)3-(1,2–O2(OH)2C6Cl4)1]·0.5CH2Cl2is demonstrated. The crystal structure was drawn at 50% probability level of ellipsoids. Solvents ofcrystallization (0.5 x CH2Cl2) are omitted for clarity. Selected bond lengths [Å]: Ta(1)–O(1) = 2.063(14);Ta(1)–O(2) = 2.102(17); Ta(1)–O(3) = 2.158(17); Ta(1)–O(4) = 1.984(15); Ta(1)–O(5) = 2.064(15); Ta(1)–O(6)= 2.024(16); Ta(1)–O(7) = 1.915(15); C(1)–O(1) = 1.31(3); C(6)–O(2) = 1.29(3); C(7)–O(3) = 1.33(3); C(12)–O(4) = 1.36(3); C(13)–O(5) = 1.36(3); C(18)–O(6) = 1.34(3); C(19)–O(7) = 1.40(3); C(24)–O(8) = 1.45(3);O(8)–H(8) = 0.82; N01X–H01X = 0.98; C(31)–N01X = 1.44(3); C(32)–N01X = 1.49(3). Selected bond angles [°]:O(1)–Ta(1)–O(2) = 73.081; O(2)–Ta(1)–O(3) = 68.435; O(3)–Ta(1)–O(4) = 76.326; O(5)–Ta(1)–O(3) = 73.071;39O(5)–Ta(1)–O(6) = 76.232; O(1)–Ta(1)–O(6) = 76.618; O(4)–Ta(1)–O(7) = 168.661; C(32)–N01X–H01X =104.689; C(31)–N01X–H01X = 104.611.There were two closest cation–anion contacts in [H(DMA)]2[2.2]. The first occuredbetween an oxygen atom from one of the chelated tetrachlorocatecholates and the acidic protonin the [H(DMA)]+ cation [O(3)···H(02T) = 1.766 Å]. The second occured between an oxygenatom from another chelated tetrachlorocatecholate ligand and the acidic proton of the second[H(DMA)]+ cation [O(6)···H(01x) = 1.769 Å]. The hydrogen bonds were both smaller than theVan der Waals radii for oxygen and hydrogen [rvdw = 2.72 Å].For the anionic part, the hydroxyl groups of the monodentate C6Cl4O(OH)- ligand moietiesformed two hydrogen bonds with two oxygen atoms from two of the chelatedtetrachlorocatecholate ligands [O(3)···H(8) = 2.078 Å; O(2)···H(8) = 2.096 Å]. The tantalum(V)center in the anion [2.2]2– exhibited a distorted pentagonal bipyramidal geometry. The axialpositions were occupied by one of the monodentate tetrachlorocatechol ligands and one of thechelated tetrachlorocatechol ligands. The O(1)–Ta–O(7) angle deviated from linearity [O(1)–Ta(1)–O(7) = 168.661°] likely due to steric repulsion or short intramolecular contacts byhydrogen bonds. Five oxygen atoms from the three chelated tetrachlorocatechol ligands occupiedthe pentagonal plane and displayed slightly perturbation from the 72° regular pentagonal angles[equatorial-equatorial: O(1)–Ta(1)–O(2) = 73.081°; O(2)–Ta(1)–O(3) = 68.435°; O(3)–Ta(1)–O(4) = 76.326°; O(5)–Ta(1)–O(3) = 73.071°; O(5)–Ta(1)–O(6) = 76.232°; O(1)–Ta(1)–O(6) =76.618°] (Figure 2.14).40Figure 2.14 Sketch of the center structure of [H(DMA)]2[2.2].The Ta–O bond distances in [H(DMA)]2[2.2] ranged from 1.915(15) Å to 2.158(17) Å. Theaverage Ta–O bond lengths of chelated tetrachlorocatechol moieties in [H(DMA)]2[2.2] [avg.Ta–O 2.066(13) Å] was longer than that of [Ta(C6H4O2)3py] [avg. 2.025(12) Å]. The averageTa–O bond lengths of monodentate tetrachlorocatechol moieties in [H(DMA)]2[2.2] [avg. Ta–O1.915(15) Å] was shorter than that in [Ph3C][Ta(OC6F5)6] [avg. 1.952(3) Å].123-128The cation moiety's measured parameters provided valuable insight into the compound'sbonding. The two H(DMA)+ cations showed slight differences in the N-CMe bond lengths(although within 3) and the N–H bonds were identical [N01X–H01X = 0.98, C(31)–N01X =1.44(3); C(32)–N01X = 1.49(3); N02T–H01X = 0.98, C(39)–N02T = 1.51(3); C(40)–N02T =1.58(4)] (Figure 2.15). The acidic proton was different from the acidic proton of Dr. Hazin’scrystal, H(OEt2)(H2O)Ta(1,2-O2C6Cl4)2(1,2–O2(OH)2C6Cl4)2, which was coordinatedasymmetrically through the oxygen atom of one Et2O and an oxygen atom of one H2O group[(10)–H(10) = 1.10(4) Å; O(11)–H(10) = 1.50(4) Å].112,12141Figure 2.15 Metrical parameters [Å] for the two cations of [H(DMA)]2[Ta-(1,2-O2C6Cl4)3-(1,2–O2(OH)2C6Cl4)]·0.5CH2Cl2.2.2.5 [H(DMA)][2.4]DMA was used instead of Et2O with the goal of forming a more stable Brønsted acid withthe Ta(V) WCA coordinated with tetrachlorocatechol ligands. In a glovebox, two Schlenk flaskswere charged with tetrachlorocatechol and TaCl5 (3:1 ratio), respectively. The flasks weretransferred to a Schlenk line and placed under a flow of nitrogen. To each was added CH2Cl2 (ca.3 mL each) and the resultant mixtures were stirred and heated by immersion in a 90 °C oil bathuntil the solid reactants dissolved. The orange tetrachlorocatechol solution was then added to thecolourless solution of TaCl5. The Schlenk flask containing the reaction mixture was equippedwith a water-cooled condenser and immersed in an oil bath heated to 90 °C for around one hour.The resultant solution was cooled to 20 °C and 0.5 mL of DMA was added. Immediately, somelight yellow crystals precipitated from the solution, which was tentatively assigned asH(DMA)[Ta(1,2-O2C6Cl4)3]. The detailed reaction route is shown in Scheme 2.4.42Scheme 2.4 Synthesis of Brønsted acid [H(DMA)]2[2.4].The isolated product was characterized by using 1H NMR spectrum at ambient temperature.The 1H NMR spectrum of [H(DMA)][2.4] in CDCl3 showed DMA signals. Given that the anion[2.4]– contained no protons, little information about the anion could be obtained from the 1HNMR spectrum. However, the N-CH3 peak and the Ph-H peak suggested that the desired[H(DMA)]+ cation contained a DMA molecule (δ =3.23, 6H, N-CH3; δ =7.42, 5H, Ph-H) (Figure2.16). The broad peak at around 9.74 ppm tentatively assigned to the acidic proton peak (δ =9.74,1H, [H(DMA)]+). The acidic proton at δ = 9.74 ppm was similar to the acidic proton inpreviously reported [H(DMA)]-rac-mer-[P-(C6H4CO2)3](δ =9.31 acetone-d6, at 25 °C).121Unfortunately, efforts to obtain additional support for the assigned structure using 13C NMRspectroscopy were unsuccessful as only weak signals were observed. Furthermore, the crystalsfrom crude product solution melted in X-ray oil and could not be analyzed crystallographically.43Figure 2.16 1H NMR (400 MHz, CD2Cl2, Room Temperature) spectrum of [H(DMA)][2.4].※ indicates impurities. ∷ indicates grease peak.2.2.6 H(THF)2[2.3]THF was used instead of Et2O with the goal of forming a more stable Brønsted acid withthe Ta(V) WCA coordinated with tetrachlorocatechol ligands. In a glovebox, two Schlenk flaskswere charged with tetrachlorocatechol and TaCl5 (4:1 ratio), respectively. The flasks weretransferred to a Schlenk line and placed under a flow of nitrogen. To each was added CH2Cl2 (ca.3 mL each) and the resultant mixtures were stirred and heated by immersion in a oil bath (90 °C)until the solid reactants dissolved. The orange tetrachlorocatechol solution was then added to thecolourless solution of TaCl5. The Schlenk flask containing the reaction mixture was equippedwith a water-cooled condenser and immersed in an oil bath heated to 90 °C for around one hour.44The resultant solution was cooled to 20 °C and 0.5 mL of THF was added. Immediately, somelight purple crystals precipitated from the solution, which was tentatively assigned asH(THF)2[Ta(1,2-O2C6Cl4)2(1,2–O2(OH)2C6Cl4)2]. The detailed reaction route is shown inScheme 2.5.Scheme 2.5 Synthesis of Brønsted acid H(THF)2[2.3].At low temperature, there were multiple THF signals, it was hypothesized that one kind wasfree THF solvent while the others were the THF molecules connected to the active proton. Thecomplicated signals may indicate that the connected THF molecules were in different microenvironments. The OCH2CH2 signals from THF were at 4.00 ppm and 4.15 ppm, while theOCH2CH2 signals from THF were at 1.81 ppm and 2.03 ppm (T = –85 °C, CD2Cl2: δ = 4.00 and4.15, OCH2CH2; δ = 1.81 and 2.03, OCH2CH2)(Figure 2.17). After analyzing, the sum of twodifferent THF signals were assigned to 2 THF molecules. Three interesting signals around 7.8ppm were tentatively assigned to the Ar-OH peaks, which may indicate that the monodentatetetrachlorocatechol may have Ar-OH in three different micro environments (T = –85 °C, CD2Cl2:δ =7.40, 8.06 and 8.96, Ar-OH). Or maybe only 7.40 ppm was the Ar-OH peak, and the othertwo peaks just indicated some impurities.45Meanwhile, the signal at 16.99 ppm was tentatively assigned to the active proton peak (T =–85 °C, CD2Cl2: δ = 16.99, H(THF)2), which was similar to the acidic proton in Brønsted acid,H(THF)2[P(1,2-O2C6Cl4)3] (T = –85 °C, CD2Cl2: δ = 16.90, H).113 Unfortunately, efforts toobtain additional support for the assigned structure using 13C NMR spectroscopy wereunsuccessful as only weak signals were observed. Furthermore, the crystals from crude productsolution melted in X-ray oil and could not be analyzed crystallographically.Figure 2.17 1H NMR (400 MHz, CD2Cl2, –85 °) spectrum of H(THF)2[2.3].※ indicates impurities. ∷ indicates grease peak.462.2.7 H(THF)2][2.4]THF was used instead of Et2O with the goal of forming a more stable Brønsted acid withthe Ta(V) WCA coordinated with tetrachlorocatechol ligands. In a glovebox, two Schlenk flaskswere charged with tetrachlorocatechol and TaCl5 (3:1 ratio), respectively. The flasks weretransferred to a Schlenk line and placed under a flow of nitrogen. To each was added CH2Cl2 (ca.3 mL each) and the resultant mixtures were stirred and heated by immersion in a 90 °C oil bathuntil the solid reactants dissolved. The orange tetrachlorocatechol solution was then added to thecolourless solution of TaCl5. The Schlenk flask containing the reaction mixture was equippedwith a water-cooled condenser and immersed in an oil bath heated to 90 °C for around one hour.The resultant solution was cooled to 20 °C and 0.5 mL of THF was added. Immediately, somelight yellow crystals precipitated from the solution, which was tentatively assigned asH(THF)2[Ta(1,2-O2C6Cl4)3]. The detailed reaction route is shown in Scheme 2.6.Scheme 2.6 Synthesis of Brønsted acid H(THF)2[2.4].The initiator was characterized at low temperature, since the room temperature did not showthe active protons. Given that the anion, [2.4]–, contains no hydroxyl group, little informationabout the anion could be obtained from the 1H NMR spectrum. However, for the cationic part,the methylene group connected to oxygen from THF had the signal at 3.67 ppm, while the other47methylene group from THF, OCH2CH2, had signals at 1.78 ppm (T = –85 °C, CD2Cl2: δ =3.67OCH2CH2; δ = 1.78 OCH2CH2)(Figure 2.18). These signals were assigned to eight THFmolecules instead of two likely due the remaining free THF solvent. Meanwhile, the small signalat 16.92 ppm was tentatively assigned to the active proton peak (T = –85 °C, CD2Cl2: δ = 16.92H(THF)2). The active proton shift was close to the acidic proton signal of previously reportedinitiator, H(THF)2[P(1,2-O2C6Cl4)3] (T = –85 °C, CD2Cl2: δ = 16.90, H(THF)2).113 Unfortunately,efforts to obtain additional support for the assigned structure using 13C NMR spectroscopy wereunsuccessful as only weak signals were observed. Furthermore, the crystals from crude productsolution melted in X-ray oil and could not be analyzed crystallographically.Figure 2.18 1H NMR (400 MHz, CD2Cl2, –85 °) spectrum of H(THF)2[2.4].482.2.8 H(Et2O)2[2.3]Et2O was used with the goal of forming a more stable Brønsted acid with the Ta(V) WCAcoordinated with tetrachlorocatechol ligands. In a glovebox, two Schlenk flasks were chargedwith tetrachlorocatechol and TaCl5 (4:1 ratio), respectively. The flasks were transferred to aSchlenk line and placed under a flow of nitrogen. To each was added CH2Cl2 (ca. 3 mL each)and the resultant mixtures were stirred and heated by immersion in a 90 °C oil bath until thesolid reactants dissolved. The orange tetrachlorocatechol solution was then added to thecolourless solution of TaCl5. The Schlenk flask containing the reaction mixture was equippedwith a water-cooled condenser and immersed in an oil bath heated to 90 °C for around one hour.The resultant solution was cooled to 20 °C and 0.5 mL of Et2O was added. Immediately, somelight green crystals precipitated from the solution, which was tentatively assigned asH(Et2O)2[Ta(1,2-O2C6Cl4)2(1,2–O2(OH)2C6Cl4)2]. The detailed reaction route is shown inScheme 2.7.Scheme 2.7 Synthesis of Brønsted acid H(OEt2)2[2.3].At low temperature, the 1H NMR spectrum of H(OEt2)2[2.3] in CD2Cl2 exhibited signalsassigned to the coordinated Et2O (T = –85 °C, CD2Cl2: δ = 4.04, 4H, OCH2CH3; 1.39, 6H,CH2CH3)(Figure 2.19). Besides, a signal was observed at 9.75 ppm which was tentatively49assigned to the hydroxyl proton of a monodentate tetrachlorocatechol moiety.(T = –85 °C,CD2Cl2: δ = 6.88, 5H, Ar-OH). A sharp signal at 16.74 ppm was tentatively assigned to theacidic proton in H(OEt2)2[2.3] which was similar to the acidic proton of similar Brønsted acid,H(OEt2)2[P(1,2-O2C6Cl4)3], (T = –85 °C, CD2Cl2: δ = 16.70, H).113 Unfortunately, efforts toobtain additional support for the assigned structure using 13C NMR spectroscopy wereunsuccessful as only weak signals were observed. Furthermore, the crystals from crudeproduct solution melted in X-ray oil and could not be analyzed crystallographically.Figure 2.19 1H NMR (400 MHz, CD2Cl2, –85 °C) spectrum of H(OEt2)2[2.3].502.2.9 H(Et2O)2[2.4]Et2O was used with the goal of forming a more stable Brønsted acid with the Ta(V) WCAcoordinated with tetrachlorocatechol ligands. In a glovebox, two Schlenk flasks were chargedwith tetrachlorocatechol and TaCl5 (3:1 ratio), respectively. The flasks were transferred to aSchlenk line and placed under a flow of nitrogen. To each was added CH2Cl2 (ca. 3 mL each)and the resultant mixtures were stirred and heated by immersion in a 90 °C oil bath until thesolid reactants dissolved. The orange tetrachlorocatechol solution was then added to thecolourless solution of TaCl5. The Schlenk flask containing the reaction mixture was equippedwith a water-cooled condenser and immersed in an oil bath heated to 90 °C for around one hour.The resultant solution was cooled to 20 °C and 0.5 mL of Et2O was added. Immediately, somelight green crystals precipitated from the solution, which was tentatively assigned asH(Et2O)2[Ta(1,2-O2C6Cl4)3]. The detailed reaction route is shown in Scheme 2.8.Scheme 2.8 Synthesis of Brønsted acid H(Et2O)2[2.4].At low temperature (–85 °C), the 1H NMR spectrum of H(OEt2)2[2.4] in CD2Cl2 exhibitedsignals assigned to the two coordinated ether molecules (T = –85 °C, CD2Cl2: δ = 4.01, 8H,OCH2CH3; 1.39, 12H, CH2CH3)(Figure 2.20). A sharp signal at 16.77 ppm was tentativelyassigned to the acidic proton in H(OEt2)2[2.4], (T = –85 °C, CD2Cl2: δ = 16.77, H(OEt2)2), which51was close to previously reported H(OEt2)2[P(1,2-O2C6Cl4)3] at 16.70 ppm (T = –85 °C, CD2Cl2: δ= 16.70, H(OEt2)2).113 No crystal formed in the crude product solution. Unfortunately, efforts toobtain additional support for the assigned structure using 13C NMR spectroscopy wereunsuccessful as only weak signals were observed. Furthermore, the crystals from crude productsolution melted in X-ray oil and could not be analyzed crystallographically.Figure 2.20 1H NMR (400 MHz, CD2Cl2, –85 °C) spectrum of H(OEt2)2[2.4].522.2.10 SummaryIn this chapter, seven cationic initiators with Ta-containing WCAs were synthesized andsome of them were characterized with confirmed formulation. Different basic solutions wereapplied to modify the Ta-containing cationic initiators, including DMA, DMF, THF and Et2O.Meanwhile, different ratios of tantalum and tetrachlorocatechol ligands were used to synthesizesome similar structure WCAs and compare the similarity and difference. And these complexeswere all characterized by NMR spectroscopy.Among these initiators mentioned above, two unprecedented heptacoordinated cationicinitiators was synthesized, H(DMF)2[2.1] and [H(DMA)]2[2.2]. The two initiators have slightlydistorted pentagonal bipyramid geometries, which were rarely existed in WCAs. X-raycrystallographic analysis gave more information about the formations of the two initiators.H(DMF)2[2.1] can be concluded as H(DMF)2[Ta(1,2-O2C6Cl4)2(1,2–O2(OH)2C6Cl4)2O=CHN(CH3)2]·0.7CH2Cl2, and [H(DMA)]2[2.2] can be concluded as[H(DMA)]2[Ta(1,2-O2C6Cl4)3-(1,2–O2(OH)2C6Cl4)1]·0.5CH2Cl2. The two heptacoordinatedWCAs provided some insights of WCAs field from new perspective.2.3 Experimental Procedures2.3.1 The General ProceduresSince these synthesized cationic initiators were sensitive to air and moisture, all theexperiments were executed under a nitrogen atmosphere. Standard Schlenk line and gloveboxwere widely applied during whole procedure.All solvents were dried and degassed before use. The oxygen and other gases dissolved inCH2Cl2 (Sigma Aldrich) were firstly removed. CH2Cl2was dried by the solvent purification53system (SPS) and, subsequently, was stored over CaH2 overnight, distilled under nitrogen, anddegassed three times. DMF and DMA from Fisher Scientific, were dried with magnesium sulfateovernight. THF was distilled from sodium/benzophenone ketyl. Et2O was dried with CaH2.These basic solvents were each distilled under nitrogen and degassed three times using thefreeze-pump-thaw method. These purified solvents were stored in Schlenk bombs with 3Åmolecular sieves in a nitrogen atmosphere for one to two weeks.Tetrachlorocatechol was synthesized following a literature procedure.137 Purified catecholwas mixed with hydrochloric acid and acetic acid, and the mixture was oxidized for 2 hours withhydrogen peroxide. The crude products were then azeotropically distilled in hot toluene usingDean-Stark distillation. After Dean-Stark distillation, solid tetrachlorocatechol was recrystallizedfrom toluene.For solid chemicals purification, different requirements were applied. Tantalumpentachloride from Sigma Aldrich was only dried with Schlenk line under vacuum. To eliminatecertain impurities, catechol (Sigma Aldrich) was recrystallized three times with toluene, and thensublimed under vacuum at 130 °C. Synthesized tetrachlorocatechol was degassed with theSchlenk line under vacuum.During the COVID-19 epidemic, mass spectrometry, NMR spectroscopy, and X-raycrystallography were carried out with the assistance of Chemistry Department staff. NMRspectra were obtained using a Bruker Avance 400 MHz spectrometer at ambient and lowtemperatures. All NMR spectra were referred to deuterated solvents such as CDCl3, CD2Cl2, andBenzene.542.3.2 The H(DMF)2[2.1] Synthesis ProcedureIn a glovebox, 0.099 g (0.28 mmol) of TaCl5 was weighed into a 10 mL Schlenk flaskequipped with a reflux condenser. The apparatus was transferred to a Schlenk line and anhydrous4 mL CH2Cl2 was added using a syringe under nitrogen. The resultant white suspension wasrigorously refluxed by immersing the flask in an oil bath that was pre-heated to 90 °C. Afterseveral minutes most of the solid had dissolved. Simultaneously, a Schlenk flask containing anorange solution of 0.272 g (1.09 mmol) of tetrachlorocatechol in 3 mL CH2Cl2 was immersed inthe same oil bath under a flow of nitrogen. Subsequently, a clean cannula equipped with a filterwas employed to rapidly transfer the tetrachlorocatechol solution onto the TaCl5 solution. Thereaction mixture was stirred for several minutes in the 90 °C oil bath whereupon the orangesolution turned into a dark yellow mixture. The reaction mixture was refluxed and stirred for atotal of 80 minutes whereupon it was removed from the oil bath. Once the reaction mixture hadcooled to room temperature, 1.5 mL of DMF was injected with a syringe. After adding DMF,some white smoke was observed above the liquid surface. To obtain a precipitate, the solutionwas cooled to room temperature and then immersed in an ice bath. A dark yellow solid wasisolated after removing the soluble portion with a cannula. This solid was dried in vacuo. Thecollected solid was washed twice with 1.5 mL CH2Cl2 and dried in vacuo for about 1 hour. Thedried product was weighed inside the glovebox; the mass was 0.250 g, 0.173 mmol, and the yieldwas 62%, based on the mass of TaCl5. A concentrated solution of the crude product in CH2Cl2was placed in the glovebox freezer (–30 °C) for three weeks where upon pale yellow crystalswere formed. The single-crystals were harvested and analyzed by X-ray crystallography to revealthe identity of the product to be H(DMF)2[Ta(1,2-O2C6Cl4)2(O=CHN(CH3)2)(1,2-O(OH)C6Cl4)2],H(DMF)2[2.1].551H NMR spectrum (400 MHz, CDCl3, 25 °C): cation: δ = 8.12 (br, 3H, O=CH), 3.03(m,18H, N(CH3)2); anion: δ = 5.83 (br, 2H, Ar-OH).1H NMR spectrum (400 MHz, CD2Cl2, –85 °C): cations: active proton: δ =19.41 (br s, 1H,H(DMF)2); 2 DMF in cation: δ =2.83 (3JHH= 10.48 Hz, q, 12H, N(CH3)2), δ =7.99 (m, 3H,O=CH); WCAs: 1DMF in anion δ =3.07 (br, 6H, N(CH3)2), δ =8.14 (m, 1H, O=CH); δ =9.16 (s,2H, Ar-OH).2.3.3 The H(DMF)2[2.1] X-ray Structure ProcedureA Bruker X8 APEX II diffractometer was used to gather the crystal structure ofH(DMF)2[2.1] using graphite monochromated Mo K radiation. The X-ray crystallographic datawere detected using a mounted glass fiber, which contained an oil-immersed single crystal. Datafrom a single crystal of H(DMF)2[2.1] were obtained with a 120-second exposure. The BrukerSAINT software suite was then used to study the crystal structure. SADABS refined this rough-handled data in order to acquire more precise results. Furthermore, fourier difference methodswere used to demonstrate the exact structure of the obtained crystals. Two DMF molecules thatexist in the crystal with slight asymmetry to stabilize the active hydrogen. Other O-H and C-Hlocations were estimated using the Bruker X8 APEX II diffractometer settings. All non-hydrogenatoms were refined with SHELXL-2015 through the Olex2.Table 2.1 X-ray crystallographic parameters forH(DMF)2[2.1].H(DMF)2[Ta(1,2-O2C6Cl4)2(O=CHN(CH3)2)(1,2O(OH)C6Cl4)2]·0.7CH2Cl2Formula C33.72H24.72Cl18.17N3O11TaDcalc./ g cm-3 1.814Formula Weight 1472.94Color Colorless56Shape plateSize/mm3 0.42×0.20×0.05T/K 100(2)Crystal System triclinicSpace Group P-1a/Å 14.7236(7)b/Å 16.9005(9)c/Å 23.3635(11)α/° 101.374(3)β/° 99.274(3)γ/° 104.015(3)V/Å3 5394.8(5)Z 4Z' 2Wavelength/Å 0.71073Radiation type MoKαθmin/° 1.281θmax/° 29.29157Measured Refl's. 29127Ind't Refl's 29127Refl's with I > 2(I) 21346Parameters 1260Restraints 1760Largest Peak 2.301Deepest Hole -1.335GooF 1.037wR2 (all data) 0.0994wR2 0.0937R1 (all data) 0.0649R1 0.0409582.3.4 The [H(DMA)]2[2.2] Synthesis ProcedureIn a glovebox, 0.099 g (0.28 mmol) of TaCl5 was weighed into a 10 mL Schlenk flaskequipped with a reflux condenser. The apparatus was transferred to a Schlenk line and anhydrous4 mL CH2Cl2 was added using a syringe under nitrogen. The resultant white suspension wasrigorously refluxed by immersing the flask in an oil bath that was pre-heated to 90 °C. Afterseveral minutes most of the solid had dissolved. Simultaneously, a Schlenk flask containing anorange solution of 0.272 g (1.09 mmol) of tetrachlorocatechol in 3 mL CH2Cl2 was immersed inthe same oil bath under a flow of nitrogen. Subsequently, a clean cannula equipped with a filterwas employed to rapidly transfer the tetrachlorocatechol solution onto the TaCl5 solution. Thereaction mixture was stirred for several minutes in the 90 °C oil bath whereupon the orangesolution turned into a dark yellow mixture. The reaction mixture was refluxed and stirred for atotal of 80 minutes whereupon it was removed from the oil bath. Once the reaction mixture hadcooled to room temperature, 1.5 mL of DMA was injected with a syringe. After adding DMA,some white smoke was observed above the liquid surface. To obtain a precipitate, the solutionwas cooled to room temperature and then immersed in an ice bath. A dark yellow solid wasisolated after removing the soluble portion with a cannula. This solid was dried in vacuo. Thecollected solid was washed twice with 1.5 mL CH2Cl2 and dried in vacuo for about 1 hour. Thedried product was weighed inside the glovebox; the mass was 0.320 g, 0.190 mmol, and the yieldwas 67%, based on the mass of TaCl5. A concentrated solution of the crude product in CH2Cl2was placed in the glovebox freezer (–30 °C) for three weeks where upon pale yellow crystalswere formed. The single-crystals were harvested and analyzed by X-ray crystallography to revealthe identity of the product to be [H(DMA)]2[Ta (1,2-O2C6Cl4)3(1,2–O2(OH)2C6Cl4)1]·0.5CH2Cl2.,[H(DMA)]2[2.2].591H NMR spectrum (400 MHz, CDCl3, 25 °C): Active Proton: δ = 10.89 (br s, 2H,H(DMA)+); DMA: δ = 7.26 (3JHH = 10.64 Hz, m, 10H, Ph-H), δ = 3.36 (br s, 12H, N(CH3)2);WCAs: Ar-OH δ = 9.79 (s, 1H, Ar-OH).1H NMR spectrum (400 MHz, CDCl3, -85 °C): Active Proton: δ = 16.74 (s, 2H,H(DMA)+); DMA: δ = 7.04 (br, 5H, Ph-H), δ = 3.41 (3JHH= 5.44 Hz, d, 6H, N(CH3)2); WCAs:Ar-OH δ = 8.06 (s, 2H, Ar-OH).2.3.5 The [H(DMA)]2[2.2] X-ray Structure ProcedureA Bruker X8 APEX II diffractometer was used to gather the crystal structure of[H(DMA)]2[2.2] using graphite-monochromated Mo K radiation. The X-ray crystallographicdata was detected using a mounted glass fiber, which contained an oil-immersed single crystal.Data from a single crystal of [H(DMA)]2[2.2] was acquired using an X-ray diffractometer for120 seconds. The Bruker SAINT software suite was then used to study the crystal structure.SADABS refined this rough-handled data in order to acquire more precise results. Furthermore,fourier difference methods were used to demonstrate the exact structure of the obtained crystals.One DMA molecule existed in the cationic part of crystal to stabilize one active hydrogen,and there were two active hydrogen. Other O-H and C-H locations were estimated using theBruker X8 APEX II diffractometer settings. All non-hydrogen atoms wererefined with SHELXL-2015 through the Olex2.Table 2.2 X-ray crystallographic parameters for [H(DMA)]2[2.2].[H(DMA)]2[Ta-cis-(1,2-O2C6Cl4)3-(1,2–O2(OH)2C6Cl4)1]·0.5CH2Cl2Empirical formula C81H51Cl35N4O16Ta260Formula weight 2938.90Temperature/K 273.15Crystal system triclinicSpace group P-1a/Å 16.065(3)b/Å 17.875(3)c/Å 19.445(3)α/° 81.125(5)β/° 69.135(4)γ/° 84.227(5)Volume/Å3 5149.3(14)Z 2ρcalcg/cm3 1.895μ/mm-1 3.095F(000) 2868.0Crystal size/mm3 0.22 × 0.15 × 0.03Radiation Mo Kα (λ = 0.71073)2θ range for data collection/°2.26 to 45.804Index ranges -17 ≤ h ≤ 17, -19 ≤ k ≤ 19, -20 ≤ l ≤ 21Reflections collected 51459Independent reflections 13986 [Rint = 0.0980, Rsigma = 0.1094]61Data/restraints/parameters 13986/1876/1248Goodness-of-fit on F2 1.393Final R indexes [I>=2σ (I)] R1 = 0.1340, wR2 = 0.3251Final R indexes [all data] R1 = 0.2081, wR2 = 0.4045Largest diff. peak/hole / e Å-3 11.22/-2.422.3.6 The H(DMA)[2.4] Synthesis ProcedureIn a glovebox, 0.099 g (0.28 mmol) of TaCl5 was weighed into a 10 mL Schlenk flaskequipped with a reflux condenser. The apparatus was transferred to a Schlenk line and anhydrous4 mL CH2Cl2 was added using a syringe under nitrogen. The resultant white suspension wasrigorously refluxed by immersing the flask in an oil bath that was pre-heated to 90 °C. Afterseveral minutes most of the solid had dissolved. Simultaneously, a Schlenk flask containing anorange solution of 0.204 g (0.84 mmol) of tetrachlorocatechol in 3 mL CH2Cl2 was immersed inthe same oil bath under a flow of nitrogen. Subsequently, a clean cannula equipped with a filterwas employed to rapidly transfer the tetrachlorocatechol solution onto the TaCl5 solution. Thereaction mixture was stirred for several minutes in the 90 °C oil bath whereupon the orangesolution turned into a dark yellow mixture. The reaction mixture was refluxed and stirred for atotal of 80 minutes whereupon it was removed from the oil bath. Once the reaction mixture hadcooled to room temperature, 1.5 mL of DMA was injected with a syringe. After adding DMA,some white smoke was observed above the liquid surface. To obtain a precipitate, the solutionwas cooled to room temperature and then immersed in an ice bath. A dark yellow solid was62isolated after removing the soluble portion with a cannula. This solid was dried in vacuo. Thecollected solid was washed twice with 1.5 mL CH2Cl2 and dried in vacuo for about 1 hour. Thedried product was weighed inside the glovebox; the mass was 0.130 g, 0.126 mmol, and the yieldwas 45%, based on the mass of TaCl5.1H NMR spectrum (400 MHz, CDCl3, 25 °C): Active Proton: δ = 9.74 (b, 1H, H(DMA));for DMA: δ = 7.42 (3JHH= 6.64 Hz, t, 5H, Ph-H), δ = 3.23 (br s, 6H, N(CH3)2).2.3.7 The H(THF)2[2.3] Synthesis ProcedureIn a glovebox, 0.099 g (0.28 mmol) of TaCl5 was weighed into a 10 mL Schlenk flaskequipped with a reflux condenser. The apparatus was transferred to a Schlenk line and anhydrous4 mL CH2Cl2 was added using a syringe under nitrogen. The resultant white suspension wasrigorously refluxed by immersing the flask in an oil bath that was pre-heated to 90 °C. Afterseveral minutes most of the solid had dissolved. Simultaneously, a Schlenk flask containing anorange solution of 0.272 g (1.09 mmol) of tetrachlorocatechol in 3 mL CH2Cl2 was immersed inthe same oil bath under a flow of nitrogen. Subsequently, a clean cannula equipped with a filterwas employed to rapidly transfer the tetrachlorocatechol solution onto the TaCl5 solution. Thereaction mixture was stirred for several minutes in the 90 °C oil bath whereupon the orangesolution turned into a dark yellow mixture. The reaction mixture was refluxed and stirred for atotal of 80 minutes whereupon it was removed from the oil bath. Once the reaction mixture hadcooled to room temperature, 1.5 mL of THF was injected with a syringe. After adding THF,some white smoke was observed above the liquid surface. To obtain a precipitate, the solutionwas cooled to room temperature and then immersed in an ice bath. A dark yellow solid wasisolated after removing the soluble portion with a cannula. This solid was dried in vacuo. The63collected solid was washed twice with 1.5 mL CH2Cl2 and dried in vacuo for about 1 hour. Thedried product was weighed inside the glovebox; the mass was 0.220 g, 0.168 mmol. And theyield was 60% based on the mass of TaCl5.1H NMR spectrum (400 MHz, CD2Cl2, -85 °C): THF, δ = 1.81 and 2.03 (m,4H,OCH2CH2); δ = 4.00 and 4.15 (m, 4H, OCH2CH2); WCAs: Ar-OH δ = 7.40, 8.06, 8.96 (s, 2H,Ar-OH); proton: δ=16.95 (s, 1H, H(THF)2).2.3.8 The H(THF)2[2.4] Synthesis ProcedureIn a glovebox, 0.099 g (0.28 mmol) of TaCl5 was weighed into a 10 mL Schlenk flaskequipped with a reflux condenser. The apparatus was transferred to a Schlenk line and anhydrous4 mL CH2Cl2 was added using a syringe under nitrogen. The resultant white suspension wasrigorously refluxed by immersing the flask in an oil bath that was pre-heated to 90 °C. Afterseveral minutes most of the solid had dissolved. Simultaneously, a Schlenk flask containing anorange solution of 0.204 g (0.84 mmol) of tetrachlorocatechol in 3 mL CH2Cl2 was immersed inthe same oil bath under a flow of nitrogen. Subsequently, a clean cannula equipped with a filterwas employed to rapidly transfer the tetrachlorocatechol solution onto the TaCl5 solution. Thereaction mixture was stirred for several minutes in the 90 °C oil bath whereupon the orangesolution turned into a dark yellow mixture. The reaction mixture was refluxed and stirred for atotal of 80 minutes whereupon it was removed from the oil bath. Once the reaction mixture hadcooled to room temperature, 1.5 mL of THF was injected with a syringe. After adding THF,some white smoke was observed above the liquid surface. To obtain a precipitate, the solutionwas cooled to room temperature and then immersed in an ice bath. A dark yellow solid wasisolated after removing the soluble portion with a cannula. This solid was dried in vacuo. The64collected solid was washed twice with 1.5 mL CH2Cl2 and dried in vacuo for about 1 hour. Thedried product was weighed inside the glovebox; the mass was 0.113 g, 0.106 mmol. And theyield was 38% based on the mass of TaCl5.1H NMR spectrum (400 MHz, CD2Cl2, -85 °C): THF, δ = 1.78 (br s, 40H, OCH2CH2); δ= 3.67 (br s, 40H, OCH2CH2); active proton: δ = 16.92 (s, 1H, H(THF)2).2.3.9 The H(Et2O)2[2.3] Synthesis ProcedureIn a glovebox, 0.099 g (0.28 mmol) of TaCl5 was weighed into a 10 mL Schlenk flaskequipped with a reflux condenser. The apparatus was transferred to a Schlenk line and anhydrous4 mL CH2Cl2 was added using a syringe under nitrogen. The resultant white suspension wasrigorously refluxed by immersing the flask in an oil bath that was pre-heated to 90 °C. Afterseveral minutes most of the solid had dissolved. Simultaneously, a Schlenk flask containing anorange solution of 0.272 g (1.09 mmol) of tetrachlorocatechol in 3 mL CH2Cl2 was immersed inthe same oil bath under a flow of nitrogen. Subsequently, a clean cannula equipped with a filterwas employed to rapidly transfer the tetrachlorocatechol solution onto the TaCl5 solution. Thereaction mixture was stirred for several minutes in the 90 °C oil bath whereupon the orangesolution turned into a dark yellow mixture. The reaction mixture was refluxed and stirred for atotal of 80 minutes whereupon it was removed from the oil bath. Once the reaction mixture hadcooled to room temperature, 1.5 mL of Et2O was injected with a syringe. After adding Et2O,some white smoke was observed above the liquid surface. To obtain a precipitate, the solutionwas cooled to room temperature and then immersed in an ice bath. A dark yellow solid wasisolated after removing the soluble portion with a cannula. This solid was dried in vacuo. Thecollected solid was washed twice with 1.5 mL CH2Cl2 and dried in vacuo for about 1 hour. The65dried product was weighed inside the glovebox; the mass was 0.220 g, 0.167 mmol. And theyield was 60% based on the mass of TaCl5.1H NMR spectrum (400 MHz, CD2Cl2, -85 °C): Et2O, δ = 4.04 (3JHH = 4.12 Hz, d, 4H,OCH2CH3); δ = 1.39 (3JHH=4.23 Hz, t, 6H, OCH2CH3); WCAs, Ar-OH, δ = 6.88 (br s, 5H, Ar-OH); active proton, δ =16.74 (s, 1H, H(OEt2)2).2.3.10 The H(Et2O)2[2.4] Synthesis ProcedureIn a glovebox, 0.099 g (0.28 mmol) of TaCl5 was weighed into a 10 mL Schlenk flaskequipped with a reflux condenser. The apparatus was transferred to a Schlenk line and anhydrous4 mL CH2Cl2 was added using a syringe under nitrogen. The resultant white suspension wasrigorously refluxed by immersing the flask in an oil bath that was pre-heated to 90 °C. Afterseveral minutes most of the solid had dissolved. Simultaneously, a Schlenk flask containing anorange solution of 0.204 g (0.84 mmol) of tetrachlorocatechol in 3 mL CH2Cl2 was immersed inthe same oil bath under a flow of nitrogen. Subsequently, a clean cannula equipped with a filterwas employed to rapidly transfer the tetrachlorocatechol solution onto the TaCl5 solution. Thereaction mixture was stirred for several minutes in the 90 °C oil bath whereupon the orangesolution turned into a dark yellow mixture. The reaction mixture was refluxed and stirred for atotal of 80 minutes whereupon it was removed from the oil bath. Once the reaction mixture hadcooled to room temperature, 1.5 mL of Et2O was injected with a syringe. After adding Et2O,some white smoke was observed above the liquid surface. To obtain a precipitate, the solutionwas cooled to room temperature and then immersed in an ice bath. A dark yellow solid wasisolated after removing the soluble portion with a cannula. This solid was dried in vacuo. Thecollected solid was washed twice with 1.5 mL CH2Cl2 and dried in vacuo for about 1 hour. The66dried product was weighed inside the glovebox; the mass was 0.170 g, 0.159 mmol. And theyield was 58% based on the mass of TaCl5.1H NMR spectrum (400 MHz, CD2Cl2, -85 °C): Et2O, δ = 4.01 (3JHH = 6.72 Hz, t, 12H,CH3); δ = 1.39 (3JHH= 6.64 Hz, d, 8H, OCH2); active proton, δ=16.77 (s, 1H, H(OEt2)2).67Chapter 3: Single Component Cationic Initiators for Olefin polymerizationwith Hexacoordinate and Heptacoordinate Niobium(V) Weakly CoordinatingAnions3.1 IntroductionCationic polymerization has a substantially faster propagation step than anionic andradical polymerization. Usually, the propagation may finish within seconds for cationicpolymerization, while anionic and radical polymerization may require minutes to hours.138–140Thus, cationic polymerization is more challenging to control because of chain transfer and earlytermination. WCAs are an excellent method to stabilize the protic initiating species and thepropagating carbocation due to their weakly coordinating nature.138 WCAs have large volumes,which can separate the negative charge around the whole anions instead of anchoring to specificpositions.63 Historically, elements from Group 13, such as boron and aluminum, were widelyused in WCAs.60,76,94,95Protons (H+) are the functional initiating species of the single component Brønsted acidinitiators, which are simple and can lessen the difficulty of the mechanism investigation.Meanwhile, those single component initiators are sometimes weighable, simplifying the precisecontrol of the monomer-to-initiator ratio. However, the single component initiators are usuallymore difficult to synthesize, isolate, and store than the binary systems. Only a few singlecomponent initiators have been synthesized and characterized until now. Prominent examplesinclude: H(OEt2)2[Al{O(CF3)3}4], H(OEt2)2[B(C6F5)4], and H(L)2[P(1,2-O2C6Cl4)3] (L= OEt2,THF, DMF). 85,86,99,100,113,141 Moreover, low stability of solid Brønsted acids may impede thedevelopment of industrial initiators for cationic polymerization. More efforts are deserved in this68area since Brønsted acid initiators show many excellent properties during polymerization. Thusthe Gates group has made a significant effort to develop Brønsted acid initiators.112,113,128The introduction of Chapter 2 showed various Brønsted acids with P-centered and Ta-centered WCAs, which were synthesized by our group previously. Meanwhile, Chapter 2introduced some initiators containing Ta-centered WCAs, modified using other solutionsconnected to protons (H+) instead of Et2O, like DMF, DMA, and aniline. Two heptacoordinatedcationic initiators were characterized and analyzed. They are H(DMF)2[Ta(1,2–O2(OH)2C6Cl4)2(1,2-O2C6Cl4)2O=CHN(CH3)2] and [H(DMA)]2[Ta(1,2–(OH)OC6Cl4)1(1,2–O2C6Cl4)3]. Initiators with Ta-containing WCAs have shown interesting properties, prompting usto examine other elements from group 5.However, when considering their industrial application, the cost of tantalum cannot beneglected, because some of those single-component initiators can not be regenerated andrecycled now. The price of tantalum is roughly $298–312 USD per kilogram.142 Thus, our goalwas to form larger WCAs using cheaper elements from Group 5, like niobium and vanadium.Vanadium prefers to form +4 complexes, making it hard to coordinate with more ligands andgenerate larger WCAs.143 Thus, vanadium may not be suitable for WCAs. However, research hasshown that niobium could form highly coordinated compounds. Besides, niobium only costs61.4–85.6 USD per kilogram, which is much cheaper than tantalum.144 Meanwhile, toxicity ofthe metals, which may be left in the polymer product, may also be a consideration. Compared totantalum, the toxicity of niobium may be less.144 Thus, niobium with bulky ligands may bepromising for producing large weakly coordinating anions. Due to the price differences, niobiummay be more suited for future commercial uses than tantalum.69Figure 3.1 Structures of traditional Nb-centered single component initiators.However, only several initiators containing niobium(V) have been reported previously.For binary initiators, only a few research showed Nb-contained systems. For example,NbCl5/6RLi/Ph3CCl and NbCl5/ethylene/glycol were used in metal coordinationpolymerization.145–147 As for Nb-related single component initiators, related researches was evenless. H[Nb(O2C6H4)3] and [Ph3C][Nb(OC6F5)6] were two notable examples as shown in Figure3.1.100,148,149 So our group member Anna Louise Bennett has made an effort to synthesized somesolid single component initiators containing niobium. Figure 3.2 shows the structures of the twosolid Brønsted acids, H(L)2[Nb(1,2-O2C6Cl4)3] and H(L)2 [Nb(1,2-O2C6Cl4)2(1,2-O(OH)C6Cl4)2](L = Et2O). Bennett also proved the two initiators are effective for polymerizing n-butyl vinylether and styrene.In Chapter 3, we will focus on four intriguing single component initiators that have beencoupled with Brønsted acid and Nb-containing WCAs, HL2[Nb(1,2-O2C6Cl4)2(1,2-O(OH)C6Cl4)2]and HL2[Nb(1,2-O2C6Cl4)3] (L = DMA, aniline). These single component initiators werecharacterized, and provided an inspiring insight into the rarely investigated Nb-centered WCAs.70Figure 3.2 Structures of Nb-centered single component initiators synthesized by our group.3.2 Results and Discussion3.2.1 Synthesis of [H(DMA)][3.1]Chapter 2 have showed several solid and weighable Brønsted acids. These initiators wereformed by reacting 1,2-(OH)2C6Cl4 with TaCl5 in CH2Cl2, then adding a basic solution, such asEt2O, DMF, DMA and THF, to stabilize the proton, H+. With different metal to ligand ratios,HL2[Ta(1,2-O2C6Cl4)3] (L = Et2O, DMF, DMA, THF) and HL2[Ta(1,2-O2C6Cl4)2(1,2-O(OH)C6Cl4)2] (L = Et2O, DMF, DMA, THF) were synthesized. The characterization resultsshowed that the size of these WCAs was quite large.These tantalum(V) anions formed large size WCAs with moderate coordinating strength.Thus, it is reasonable to expect that WCAs with niobiumwill also be able to stabilize strongBrønsted acids and generate functional cationic initiators in the future. Meanwhile, it isreasonable to hypothesize that these initiators will function effectively as cationic initiators fordifferent monomers.71DMA was used instead of Et2O with the goal of forming a more stable Brønsted acid withthe Nb(V) WCA coordinated with tetrachlorocatechol ligands. In a glovebox, two Schlenk flaskswere charged with tetrachlorocatechol and NbCl5 (4:1 ratio), respectively. The flasks weretransferred to a Schlenk line and placed under a flow of nitrogen. To each was added CH2Cl2 (ca.3 mL each) and the resultant mixtures were stirred and heated by immersion in a oil bath (90 °C)until the solid reactants dissolved. The orange tetrachlorocatechol solution was then added to theorange solution of NbCl5. The Schlenk flask containing the reaction mixture was equipped with awater-cooled condenser and immersed in an oil bath heated to 90 °C for around one hour. Theresultant solution was cooled to 20 °C and 0.5 mL of DMA was added. The product tentativelyassigned as H(DMA)[Nb(1,2-O2C6Cl4)2(1,2–O2(OH)2C6Cl4)2]. The detailed reaction route isshown in Scheme 3.1.Scheme 3.1 Synthesis of Brønsted acid H(DMA)[3.1].The isolated product was characterized by 1H NMR spectroscopy at ambient temperatureand the spectrum is shown in Figure 3.3. The 1H NMR spectrum of [H(DMA)][3.1] in CDCl3exhibited one DMA molecule (δ =3.31, 6H, N(CH3)2; δ =7.47 and 7.71, 5H, Ph-H)(Figure 3.3).And the hydrogen atoms of the hydroxyl groups of the two monodentate tetrachlorocatechol inWCA, [3.1]-, had a shift at 5.97 ppm. More interestingly, the 1H NMR spectrum displayed a72signal around 13 ppm as the active proton signal at ambient temperature (δ = 13.06, T = 20 °C,CDCl3).Figure 3.3 1H NMR (400 MHz, CD2Cl2, Room Temperature) spectrum of H(DMA)[3.1].※ indicates impurities. ∷ indicates grease peak.The proton peak in 13.06 ppm is similar to the acidic proton in [H(DMA)]-rac-mer-[P-(C6H4CO2)3] (δ =9.31 acetone-d6, at 25 °C) synthesized by our group member, Khatera Hazin.121Besides, the proton peak in 13.06 ppm was also close to the shift of H(OEt2)2[P(1,2-O2C6Cl4)3] at16.70 ppm.113 Besides data from our group, there were other experimental data about the protonin related Brønsted acids to compare. The proton peak of H(OEt2)2[B(C6F5)4] was around 15.573ppm at room temperature (δ = 15.5, CD2Cl2 at 25 °C).129 The other traditional B-containingWCAs, HL2[B(CF3)4](L=Et2O), showed similar shift of proton around 16 ppm (δ = 16.3, CD2Cl2at 25 °C).130,131 Brønsted acids with Al-containing and P-containing WCAs also showed similarproton shift, H(OEt2)2[Al{OC(CF3)3}4] (δ = 14.7, C6D6, at 25 °C) and H(OEt2)2[P(C2O4)3] (δ =15.5, CDCl3, at 25 °C).133,134 When it came to some N-containing WCAs, some research showedthat the proton signals were all close to 16.5 ppm.135,1363.2.2 Synthesis of [H(aniline)][3.1]Aniline was used instead of Et2O with the goal of forming a more stable Brønsted acid withthe Nb(V) WCA coordinated with tetrachlorocatechol ligands. In a glovebox, two Schlenk flaskswere charged with tetrachlorocatechol and NbCl5 (4:1 ratio), respectively. The flasks weretransferred to a Schlenk line and placed under a flow of nitrogen. To each was added CH2Cl2 (ca.3 mL each) and the resultant mixtures were stirred and heated by immersion in a oil bath (90 °C)until the solid reactants dissolved. The orange tetrachlorocatechol solution was then added to theorange solution of NbCl5. The Schlenk flask containing the reaction mixture was equipped with awater-cooled condenser and immersed in an oil bath heated to 90 °C for around one hour. Theresultant solution was cooled to 20 °C and 0.5 mL of aniline was added. The product wastentatively assigned as H(aniline)[Nb(1,2-O2C6Cl4)2(1,2-O2(OH)2C6Cl4)2]. The detailed reactionroute is shown in Scheme 3.2.74Scheme 3.2 Synthesis of Brønsted acid H(aniline)[3.1].The isolated product was characterized with 1H NMR spectrum at ambient temperature asshown in Figure 3.4. The 1H NMR spectrum of [H(aniline)][3.1] in CDCl3 exhibited the ratiobetween the peaks of Ph-H and the peaks of NH3+ signals from aniline were roughly 5:3,indicating that one proton may couple with one aniline molecule (δ = 4.86, 6H, NH3+; δ = 6.77and 7.19, 10H, Ph-H, T = 25 °C, CDCl3). It was difficult to determine the active proton shiftsince it formed NH3+ instead of a separated active proton. However, the broad instead of sharpsinglet NH3+ peak indicated that the active proton may attached to the aniline molecule (δ = 4.86,T = 25 °C, CDCl3). And the hydrogen atoms of the hydroxyl groups of the two monodentatetetrachlorocatechol in WCA, [3.1]-, had a shift at 5.97 ppm. And the hydrogen atoms of thehydroxyl groups of the two monodentate tetrachlorocatechol in WCA, [3.1]-, had a shift at 5.30ppm (δ = 5.30, 1H, Ar-OH)).75Figure 3.4 1H NMR (400 MHz, CD2Cl2, Room Temperature) spectrum of H(aniline)[3.1].※ indicates impurities. ∷ indicates grease peak.3.2.3 Synthesis of [H(DMA)][3.2]DMA was used instead of Et2O with the goal of forming a more stable Brønsted acid withthe Nb(V) WCA coordinated with tetrachlorocatechol ligands. In a glovebox, two Schlenk flaskswere charged with tetrachlorocatechol and NbCl5 (3:1 ratio), respectively. The flasks weretransferred to a Schlenk line and placed under a flow of nitrogen. To each was added CH2Cl2 (ca.3 mL each) and the resultant mixtures were stirred and heated by immersion in a oil bath (90 °C)until the solid reactants dissolved. The orange tetrachlorocatechol solution was then added to the76orange solution of NbCl5. The Schlenk flask containing the reaction mixture was equipped with awater-cooled condenser and immersed in an oil bath heated to 90 °C for around one hour. Theresultant solution was cooled to 20 °C and 0.5 mL of DMA was added. The product wastentatively assigned as H(DMA)[Nb(1,2-O2C6Cl4)3]. The detailed reaction route is shown inScheme 3.3.Scheme 3.3 Synthesis of Brønsted acid H(DMA)[3.2].The isolated product was characterized with 1H NMR spectrum at ambient temperature asshown in Figure 3.5. Given that the anion [3.2]– contains no protons, little information about theanion could be obtained from the 1H NMR spectrum. The 1H NMR spectrum of [H(DMA)][3.2]in CDCl3 exhibited signals of the cationic part. The H peaks of the Ph-N-(CH3)2 and Ph-H groupcould be confirmed as 6:5 (δ =3.35, 6H, N(CH3)2; δ =7.45 and 7.71, 5H, Ph-H, T = 25 °C,CDCl3). The active proton peak was shown around 13 ppm (δ =12.98, H(DMA), T = 25 °C,CDCl3), even though the signal was broad and not significant. The acidic proton at δ = 12.98ppm is similar to the acidic proton in [H(DMA)]-rac-mer-[P-(C6H4CO2)3] (δ =9.31 acetone-d6, at25 °C) synthesized by our group member, Khatera Hazin.121 Besides, the proton peak in 12.98ppm was also close to the shift of H(OEt2)2[P(1,2-O2C6Cl4)3] was 16.70 ppm.113 And the 12.9877ppm is also similar to the Brønsted acids with carborane anions, H(OEt2)2[CHB11Me5Cl6] (δ =13.8, C6D6, at 25 °C) and H(OEt2)2[CHB11Me5Br6] (δ = 11.7, C6D6, at 25 °C).132Figure 3.5 1H NMR (400 MHz, CD2Cl2, Room Temperature) spectrum of H(DMA)[3.2].※ indicates impurities. ∷ indicates grease peak.3.2.4 Synthesis of [H(aniline)][3.2]Aniline was used instead of Et2O with the goal of forming a more stable Brønsted acid withthe Nb(V) WCA coordinated with tetrachlorocatechol ligands. In a glovebox, two Schlenk flaskswere charged with tetrachlorocatechol and NbCl5 (3:1 ratio), respectively. The flasks weretransferred to a Schlenk line and placed under a flow of nitrogen. To each was added CH2Cl2 (ca.783 mL each) and the resultant mixtures were stirred and heated by immersion in a oil bath (90 °C)until the solid reactants dissolved. The orange tetrachlorocatechol solution was then added to theorange solution of NbCl5. The Schlenk flask containing the reaction mixture was equipped with awater-cooled condenser and immersed in an oil bath heated to 90 °C for around one hour. Theresultant solution was cooled to 20 °C and 0.5 mL of aniline was added. The product wastentatively assigned as H(aniline)2[Nb(1,2-O2C6Cl4)3]. The reaction route is shown in Scheme 3.4.Scheme 3.4 Synthesis of Brønsted acid H(aniline)[3.2].The isolated product was characterized with 1H NMR spectrum at ambient temperature asshown in Figure 3.6. Given that the anion [3.2]– contains no protons, little information about theanion could be obtained from the 1H NMR spectrum. The 1H NMR spectrum of [H(aniline)][3.2]in CDCl3 exhibited signals of the cationic part. The H peaks of the Ph-NH3+ and Ph-H groupcould be confirmed as 3:5 (δ =5.26, 3H, Ph-NH3+; δ =6.87 and 7.22, 5H, Ph-H, T = 25 °C,CDCl3). It was difficult to determine the active proton shift since it formed NH3+ instead of aseparated active proton. However, the broad instead of sharp singlet NH3+ peak indicated that theactive proton may attached to the aniline molecule (δ =5.26, 3H, Ph-NH3+ T = 25 °C, CDCl3).79Figure 3.6 1H NMR (400 MHz, CD2Cl2, Room Temperature) spectrum of H(aniline)[3.2].※ indicates impurities. ∷ indicates grease peak.3.2.5 SummaryFour different solid and weighable Brønsted acid initiators were synthesized with Nb-centered WCAs. For the cationic part, the N-containing solutions were applied to stabilize theactive proton, like DMA and aniline. For the anionic part, two different WCAs, [3.1]- and [3.2]-were formed with different NbCl5 and tetrachlorocatechol ratio. More specifically, four Brønstedacids [H(DMA)][3.1], [H(aniline)][3.1], [H(DMA)][3.2] and [H(aniline)][3.2] were synthesizedand characterized in this chapter.80All the cationic initiators were characterized by NMR spectroscopy. And the fourdifferent cationic initiators all showed the peaks of active protons. However, the chemical shiftsvaried differently. The variations were related to basic solvent used. For DMA, the shifts werearound 13 ppm. For aniline, the active proton is connected to the -NH2 and formed NH3+ group,thus the active proton peaks were closed to the peak of -NH2 in aniline molecule which werearound 5 ppm. Meanwhile, the shift of active proton peaks also have relationships with theWCAs. The Brønsted acid initiators with 1:4 ratio Nb-containing WCAs all had larger protonshifts than the ones with 1:3 ratio niobium WCAs, regardless of the basic solvent used.3.3 Experimental3.3.1 The General ProceduresSince these synthesized cationic initiators were sensitive to air and moisture, all theexperiments were executed under a nitrogen atmosphere. Standard Schlenk line and gloveboxwere widely applied during whole procedure. All solvents were dried and degassed before use.The oxygen and other gases dissolved in CH2Cl2 (Sigma Aldrich) were firstly removed. ThenCH2Cl2 was dried by the solvent purification system (SPS). The water in the CH2Cl2 wasliberated to a great amount after passing through SPS. CH2Cl2 was dried by the solventpurification system (SPS) and, subsequently, was stored over CaH2 overnight, distilled undernitrogen, and degassed three times. DMA and aniline (Fisher Scientific), were dried withmagnesium sulfate overnight to eliminate water. These basic solvents were distilled undernitrogen and degassed three times using the freeze-pump-thaw method. These purified solventswere stored in Schlenk bombs with 3Å molecular sieves in a nitrogen atmosphere for around oneto two weeks.81Tetrachlorocatechol was synthesized following a literature procedure.137 Purified catecholwas mixed with hydrochloric acid and acetic acid, and the mixture was oxidized for 2 hourswith hydrogen peroxide. The crude products were then azeotropically distilled in hot tolueneusing Dean-Stark distillation. After Dean-Stark distillation, solid tetrachlorocatechol wasrecrystallized from toluene. For solid chemicals purification, different requirements were applied.Niobium pentachloride from Sigma Aldrich was only dried with Schlenk line under vacuum. Toeliminate certain impurities, catechol (Sigma Aldrich) was recrystallized three times with toluene,and then sublimed under vacuum at 130 °C. Synthesized tetrachlorocatechol was degassed withthe Schlenk line under vacuum.During the COVID-19 epidemic, mass spectrometry, NMR spectra, and X-raycrystallography were carried out with the assistance of Chemistry Department staff. NMRspectra were obtained using a Bruker Avance 400 MHz spectrometer at ambient and lowtemperatures. All NMR spectra were referred to deuterated solvents such as CDCl3, CD2Cl2, andBenzene.3.3.2 Synthesis of H(DMA)[3.1]In a glovebox, 0.075 g (0.28 mmol) of NbCl5 was weighed into a 10 mL Schlenk flaskequipped with a reflux condenser. The apparatus was transferred to a Schlenk line and anhydrous4 mL CH2Cl2 was added using a syringe under nitrogen. The resultant white suspension wasrigorously refluxed by immersing the flask in an oil bath that was pre-heated to 90 °C. Afterseveral minutes most of the solid had dissolved. Simultaneously, a Schlenk flask containing anorange solution of 0.272 g (1.09 mmol) of tetrachlorocatechol in 3 mL CH2Cl2 was immersed inthe same oil bath under a flow of nitrogen. Subsequently, a clean cannula equipped with a filter82was employed to rapidly transfer the tetrachlorocatechol solution onto the NbCl5 solution. Thereaction mixture was stirred for several minutes in the 90 °C oil bath whereupon the orangesolution turned into a dark yellow mixture. The reaction mixture was refluxed and stirred for atotal of 80 minutes whereupon it was removed from the oil bath. Once the reaction mixture hadcooled to room temperature, 1.5 mL of DMA was injected with a syringe. After adding DMA,some white smoke was observed above the liquid surface. To obtain a precipitate, the solutionwas cooled to room temperature and then immersed in an ice bath. A dark yellow solid wasisolated after removing the soluble portion with a cannula. This solid was dried in vacuo. Thecollected solid was washed twice with 1.5 mL CH2Cl2 and dried in vacuo for about 1 hour. Thedried product was weighed inside the glovebox; the mass was 0.150 g, 0.126 mmol. And theyield was 45% based on the mass of NbCl5.1H NMR spectrum (400 MHz, CDCl3, 25 °C): Active Proton δ = 13.06 (b, 1H, H(DMA));DMA Ph-H: δ = 7.47 (3JHH= 7.48 Hz, d, 3H) and 7.71 (3JHH= 5.56 Hz, d, 2H) (5H, Ph-H); δ =3.31 (s, 6H, N(CH3)2); for WCA, δ = 5.97 (br s, 1H, Ar-OH).3.3.3 Synthesis of H(aniline)[3.1]In a glovebox, 0.075 g (0.28 mmol) of NbCl5 was weighed into a 10 mL Schlenk flaskequipped with a reflux condenser. The apparatus was transferred to a Schlenk line and anhydrous4 mL CH2Cl2 was added using a syringe under nitrogen. The resultant white suspension wasrigorously refluxed by immersing the flask in an oil bath that was pre-heated to 90 °C. Afterseveral minutes most of the solid had dissolved. Simultaneously, a Schlenk flask containing anorange solution of 0.272 g (1.09 mmol) of tetrachlorocatechol in 3 mL CH2Cl2 was immersed inthe same oil bath under a flow of nitrogen. Subsequently, a clean cannula equipped with a filter83was employed to rapidly transfer the tetrachlorocatechol solution onto the NbCl5 solution. Thereaction mixture was stirred for several minutes in the 90 °C oil bath whereupon the orangesolution turned into a dark yellow mixture. The reaction mixture was refluxed and stirred for atotal of 80 minutes whereupon it was removed from the oil bath. Once the reaction mixture hadcooled to room temperature, 1.5 mL of aniline was injected with a syringe. After adding aniline,some white smoke was observed above the liquid surface. To obtain a precipitate, the solutionwas cooled to room temperature and then immersed in an ice bath. A dark yellow solid wasisolated after removing the soluble portion with a cannula. This solid was dried in vacuo. Thecollected solid was washed twice with 1.5 mL CH2Cl2 and dried in vacuo for about 1 hour. Thedried product was weighed inside the glovebox; the mass was 0.129 g, 0.107 mmol. And theyield was 38% based on the mass of NbCl5.1H NMR spectrum (400 MHz, CDCl3, 25 °C): aniline δ = 6.77 (3JHH = 3.12 Hz, d, 3H)and 7.19 (3JHH = 7.52 Hz, t, 3H) (m, 10H, Ph-H), amine group with the active proton (NH3+): δ= 4.86 (br, 6H, NH3+); for WCA Ar-OH: δ = 5.30 (s, 1H, Ar-OH).3.3.4 Synthesis of H(DMA)[3.2]In a glovebox, 0.075 g (0.28 mmol) of NbCl5 was weighed into a 10 mL Schlenk flaskequipped with a reflux condenser. The apparatus was transferred to a Schlenk line and anhydrous4 mL CH2Cl2 was added using a syringe under nitrogen. The resultant white suspension wasrigorously refluxed by immersing the flask in an oil bath that was pre-heated to 90 °C. Afterseveral minutes most of the solid had dissolved. Simultaneously, a Schlenk flask containing anorange solution of 0.204 g (0.82 mmol) of tetrachlorocatechol in 3 mL CH2Cl2 was immersed inthe same oil bath under a flow of nitrogen. Subsequently, a clean cannula equipped with a filter84was employed to rapidly transfer the tetrachlorocatechol solution onto the NbCl5 solution. Thereaction mixture was stirred for several minutes in the 90 °C oil bath whereupon the orangesolution turned into a dark yellow mixture. The reaction mixture was refluxed and stirred for atotal of 80 minutes whereupon it was removed from the oil bath. Once the reaction mixture hadcooled to room temperature, 1.5 mL of DMA was injected with a syringe. After adding DMA,some white smoke was observed above the liquid surface. To obtain a precipitate, the solutionwas cooled to room temperature and then immersed in an ice bath. A dark yellow solid wasisolated after removing the soluble portion with a cannula. This solid was dried in vacuo. Thecollected solid was washed twice with 1.5 mL CH2Cl2 and dried in vacuo for about 1 hour. Thedried product was weighed inside the glovebox; the mass was 0.089 g, 0.093 mmol. And theyield was 33% based on the mass of NbCl5.1H NMR spectrum (400 MHz, CDCl3, 25 °C): Active Proton δ = 12.98 (br s, 1H,H(DMA)); for DMA δ = 7.45 (3JHH = 9.44 Hz, q) and7.71 (3JHH = 6.16 Hz, d) (m, 5H, Ph-H), δ= 3.35 (s, 6H, N(CH3)2).3.3.5 Synthesis of H(aniline)[3.2]In a glovebox, 0.075 g (0.28 mmol) of NbCl5 was weighed into a 10 mL Schlenk flaskequipped with a reflux condenser. The apparatus was transferred to a Schlenk line and anhydrous4 mL CH2Cl2 was added using a syringe under nitrogen. The resultant white suspension wasrigorously refluxed by immersing the flask in an oil bath that was pre-heated to 90 °C. Afterseveral minutes most of the solid had dissolved. Simultaneously, a Schlenk flask containing anorange solution of 0.204 g (0.82 mmol) of tetrachlorocatechol in 3 mL CH2Cl2 was immersed inthe same oil bath under a flow of nitrogen. Subsequently, a clean cannula equipped with a filter85was employed to rapidly transfer the tetrachlorocatechol solution onto the NbCl5 solution. Thereaction mixture was stirred for several minutes in the 90 °C oil bath whereupon the orangesolution turned into a dark yellow mixture. The reaction mixture was refluxed and stirred for atotal of 80 minutes whereupon it was removed from the oil bath. Once the reaction mixture hadcooled to room temperature, 1.5 mL of aniline was injected with a syringe. After adding aniline,some white smoke was observed above the liquid surface. To obtain a precipitate, the solutionwas cooled to room temperature and then immersed in an ice bath. A dark yellow solid wasisolated after removing the soluble portion with a cannula. This solid was dried in vacuo. Thecollected solid was washed twice with 1.5 mL CH2Cl2 and dried in vacuo for about 1 hour. Thedried product was weighed inside the glovebox; the mass was 0.078 g, 0.085 mmol. And theyield was 30% based on the mass of NbCl5.1H NMR spectrum (400 MHz, CDCl3, 25 °C): for aniline δ = 6.87 (3JHH= 9.27 Hz, d)and 7.22 (3JHH = 7.00 Hz, t) (m, 5H, Ph-H), δ = 5.26 (br, 3H, NH3+).86Chapter 4: Conclusion4.1 IntroductionBrønsted acids are efficient single component initiators for cationic polymerization withclear functional cations, HLx+. However, Brønsted acids are usually highly active initiators,which may finish polymerization within seconds. High reaction rate may cause side reactions orearly termination. Thus, well-designed counterion were required to stabilize Brønsted acids witha moderate coordination force. Those well-designed cationic initiators can minimize chaintransfer reaction and premature termination, thus, these initiators can consequently produce highmolecular weight polymers with low dispersity. Even though main group elements like boron,aluminum and phosphorus are wildly used for WCAs, few transition metals have been tried forthe central element of WCAs. Group 5-based WCAs show similar properties of P-centeredWCAs, and are promising counterions for Brønsted acids due to their weakly coordinating nature.Thus, this thesis demonstrated some Brønsted acids with Ta(V)- or Nb(V)-containing WCAs,which all have cations, HLx+ (L = DMF, DMA, aniline, THF and Et2O, x =1 or 2). However, theTa(V)- or Nb(V)-centered WCAs are also different from traditional boron and aluminum WCAssince they introduce metal-oxygen bonds between the center element and the halogen derivativeligands. We hypothesized that Brønsted acids with Ta(V)- or Nb(V)-centered WCAs could formsolid and weighable cationic initiators.4.2 Group 5 Hexacoordinate and Heptacoordinated WCAsChapter 2 showed seven different solid Brønsted acids containing Ta-centered WCAs,H(DMF)2[2.1], [H(DMA)]2[2.2], H(DMA)2[2.4], H(THF)2[2.3], H(THF)2[2.4], H(Et2O)2[2.3]87and H(Et2O)2[2.4]. [2.1]- and [2.2]2- were two heptacoordinated WCAs, while [2.3]- and [2.4]-were two hexacoordinated WCAs. [2.3]- was the WCA with the 1:4 ratio of TaCl5 totetrachlorocatechol, while [2.4]-with the 1:3 ratio. X-ray crystallography showed theheptacoordinated structures of [2.1]- and [2.2]2-. Specifically, H(DMF)2[2.1] wasH(DMF)2[Ta(1,2–O2(OH)2C6Cl4)2 (1,2-O2C6Cl4)2(O=CHN(CH3)2)]·0.7CH2Cl2, and [H(DMA)]2[2.2] was [H(DMA)]2[Ta(1,2–(OH)OC6Cl4)1 (1,2–O2C6Cl4)3]·0.5CH2Cl2. H(DMF)2[2.1] had twochelated tetrachlorocatechol ligands, two monodentate tetrachlorocatechol ligands and anotherDMF molecule as a 2-electron donor ligand connected to the tantalum center, and the overallcharge of this WCA was -1. Thus, H(DMF)2[2.1] only had one active proton, which wasaccompanied by two DMF molecules. However, [H(DMA)]2[2.2] was different. [H(DMA)]2[2.2]had three chelated tetrachlorocatechol and only one monodentate tetrachlorocatechol. Thus, the[2.2]2- WCA had a -2 overall charge. And there were two active protons in the whole molecule.Meanwhile, the geometric structure of the two cationic initiators were both slightly distortedpentagonal bipyramidal. Besides the WCAs, the NMR spectroscopy showed that the activeproton peaks shifted to different levels when applied different solvents (DMF, DMA, THF, Et2O).Chapter 3 introduced four cationic initiators with Nb-centered WCAs which are desirablesince niobium is a less expensive element with a high coordination number from the same groupas tantalum. The four WCAs were created with different ratios of NbCl5 and tetrachlorocatecholand different solutions. DMA and aniline were used to stabilize the active proton and generateHL+. H(DMA)[3.1], H(aniline)[3.1], H(DMA)[3.2], and H(aniline)[3.2] were the four distinctcationic initiators. The NMR spectrum was used to characterize these initiators, and massspectrometry was used to investigate the two different WCAs.88This thesis mainly focused on investigating and modifying eleven Brønsted acids withTa(V)- or Nb(V)-centered WCAs. More work needs to be done to fully describe the sketch ofthese Ta(V)- or Nb(V)-centered WCAs. 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Acta 1997, 263 (1–2), 195–200.https://doi.org/10.1016/s0020-1693(97)05648-x.110AppendixFigure A.1 1H–1H COSY NMR (400 MHz for 1H, CDCl3, 25 °C) spectrum of [H(DMA)]2[2.2].111Figure A.2 LIFDI mass spectrum of [3.1]- part A.112Figure A.3 LIFDI mass spectrum of [3.1]- part B.113Figure A.4 LIFDI mass spectrum of [3.2]- part A.114Figure A.5 LIFDI mass spectrum of [3.2]- part B.115Figure A.6 13C{1H} NMR (400 MHz, CDCl3, 25 °C) spectrum of H(DMA)[3.2].116Figure A.7 1H–1H COSY NMR (400 MHz for 1H, CDCl3, 25 °C) spectrum of [H(DMA)][3.1].117Figure A.8 1H–1H COSY NMR (400 MHz for 1H, CDCl3, 25 °C) spectrum of [H(DMA)][3.2].
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Chemistry
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Gates, Derek P.
Brønsted acids with weakly coordinating anions featuring group 5 metals
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